feat: User-Interface I18n System (#728)

## Summary

**What is the goal of this PR?**
This PR introduces Internationalization (i18n) support, enabling users
to switch the UI language dynamically.

**What changes are included?**
- Core Logic: Added I18n class (`lib/I18n/I18n.h/cpp`) to manage
language state and string retrieval.

- Data Structures:

- `lib/I18n/I18nStrings.h/cpp`: Static string arrays for each supported
language.
  - `lib/I18n/I18nKeys.h`: Enum definitions for type-safe string access.
  - `lib/I18n/translations.csv`: single source of truth. 

- Documentation: Added `docs/i18n.md` detailing the workflow for
developers and translators.

- New Settings activity:
`src/activities/settings/LanguageSelectActivity.h/cpp`

## Additional Context

This implementation (building on concepts from #505) prioritizes
performance and memory efficiency.

The core approach is to store all localized strings for each language in
dedicated arrays and access them via enums. This provides O(1) access
with zero runtime overhead, and avoids the heap allocations, hashing,
and collision handling required by `std::map` or `std::unordered_map`.

The main trade-off is that enums and string arrays must remain perfectly
synchronized—any mismatch would result in incorrect strings being
displayed in the UI.

To eliminate this risk, I added a Python script that automatically
generates `I18nStrings.h/.cpp` and `I18nKeys.h` from a CSV file, which
will serve as the single source of truth for all translations. The full
design and workflow are documented in `docs/i18n.md`.

### Next Steps

- [x] Python script `generate_i18n.py` to auto-generate C++ files from
CSV
- [x] Populate translations.csv with initial translations.

Currently available translations: English, Español, Français, Deutsch,
Čeština, Português (Brasil), Русский, Svenska.
Thanks, community!

**Status:** EDIT: ready to be merged.

As a proof of concept, the SPANISH strings currently mirror the English
ones, but are fully uppercased.

---

### AI Usage

Did you use AI tools to help write this code? _**< PARTIALLY >**_
I used AI for the black work of replacing strings with I18n references
across the project, and for generating the documentation. EDIT: also
some help with merging changes from master.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: yeyeto2788 <juanernestobiondi@gmail.com>

src/activities/browser/OpdsBookBrowserActivity.cpp

@@ -2,6 +2,7 @@
 
 #include <Epub.h>
 #include <GfxRenderer.h>
+#include <I18n.h>
 #include <Logging.h>
 #include <OpdsStream.h>
 #include <WiFi.h>
@@ -28,7 +29,7 @@ void OpdsBookBrowserActivity::onEnter() {
   currentPath = "";  // Root path - user provides full URL in settings
   selectorIndex = 0;
   errorMessage.clear();
-  statusMessage = "Checking WiFi...";
+  statusMessage = tr(STR_CHECKING_WIFI);
   requestUpdate();
 
   // Check WiFi and connect if needed, then fetch feed
@@ -60,7 +61,7 @@ void OpdsBookBrowserActivity::loop() {
         // WiFi connected - just retry fetching the feed
         LOG_DBG("OPDS", "Retry: WiFi connected, retrying fetch");
         state = BrowserState::LOADING;
-        statusMessage = "Loading...";
+        statusMessage = tr(STR_LOADING);
         requestUpdate();
         fetchFeed(currentPath);
       } else {
@@ -141,11 +142,11 @@ void OpdsBookBrowserActivity::render(Activity::RenderLock&&) {
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
 
-  renderer.drawCenteredText(UI_12_FONT_ID, 15, "OPDS Browser", true, EpdFontFamily::BOLD);
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, tr(STR_OPDS_BROWSER), true, EpdFontFamily::BOLD);
 
   if (state == BrowserState::CHECK_WIFI) {
     renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, statusMessage.c_str());
-    const auto labels = mappedInput.mapLabels("« Back", "", "", "");
+    const auto labels = mappedInput.mapLabels(tr(STR_BACK), "", "", "");
     GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
     renderer.displayBuffer();
     return;
@@ -153,23 +154,23 @@ void OpdsBookBrowserActivity::render(Activity::RenderLock&&) {
 
   if (state == BrowserState::LOADING) {
     renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, statusMessage.c_str());
-    const auto labels = mappedInput.mapLabels("« Back", "", "", "");
+    const auto labels = mappedInput.mapLabels(tr(STR_BACK), "", "", "");
     GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
     renderer.displayBuffer();
     return;
   }
 
   if (state == BrowserState::ERROR) {
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 20, "Error:");
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 20, tr(STR_ERROR_MSG));
     renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 10, errorMessage.c_str());
-    const auto labels = mappedInput.mapLabels("« Back", "Retry", "", "");
+    const auto labels = mappedInput.mapLabels(tr(STR_BACK), tr(STR_RETRY), "", "");
     GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
     renderer.displayBuffer();
     return;
   }
 
   if (state == BrowserState::DOWNLOADING) {
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 40, "Downloading...");
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 40, tr(STR_DOWNLOADING));
     renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 10, statusMessage.c_str());
     if (downloadTotal > 0) {
       const int barWidth = pageWidth - 100;
@@ -184,15 +185,15 @@ void OpdsBookBrowserActivity::render(Activity::RenderLock&&) {
 
   // Browsing state
   // Show appropriate button hint based on selected entry type
-  const char* confirmLabel = "Open";
+  const char* confirmLabel = tr(STR_OPEN);
   if (!entries.empty() && entries[selectorIndex].type == OpdsEntryType::BOOK) {
-    confirmLabel = "Download";
+    confirmLabel = tr(STR_DOWNLOAD);
   }
-  const auto labels = mappedInput.mapLabels("« Back", confirmLabel, "", "");
+  const auto labels = mappedInput.mapLabels(tr(STR_BACK), confirmLabel, "", "");
   GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
 
   if (entries.empty()) {
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, "No entries found");
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, tr(STR_NO_ENTRIES));
     renderer.displayBuffer();
     return;
   }
@@ -227,7 +228,7 @@ void OpdsBookBrowserActivity::fetchFeed(const std::string& path) {
   const char* serverUrl = SETTINGS.opdsServerUrl;
   if (strlen(serverUrl) == 0) {
     state = BrowserState::ERROR;
-    errorMessage = "No server URL configured";
+    errorMessage = tr(STR_NO_SERVER_URL);
     requestUpdate();
     return;
   }
@@ -241,7 +242,7 @@ void OpdsBookBrowserActivity::fetchFeed(const std::string& path) {
     OpdsParserStream stream{parser};
     if (!HttpDownloader::fetchUrl(url, stream)) {
       state = BrowserState::ERROR;
-      errorMessage = "Failed to fetch feed";
+      errorMessage = tr(STR_FETCH_FEED_FAILED);
       requestUpdate();
       return;
     }
@@ -249,7 +250,7 @@ void OpdsBookBrowserActivity::fetchFeed(const std::string& path) {
 
   if (!parser) {
     state = BrowserState::ERROR;
-    errorMessage = "Failed to parse feed";
+    errorMessage = tr(STR_PARSE_FEED_FAILED);
     requestUpdate();
     return;
   }
@@ -260,7 +261,7 @@ void OpdsBookBrowserActivity::fetchFeed(const std::string& path) {
 
   if (entries.empty()) {
     state = BrowserState::ERROR;
-    errorMessage = "No entries found";
+    errorMessage = tr(STR_NO_ENTRIES);
     requestUpdate();
     return;
   }
@@ -275,7 +276,7 @@ void OpdsBookBrowserActivity::navigateToEntry(const OpdsEntry& entry) {
   currentPath = entry.href;
 
   state = BrowserState::LOADING;
-  statusMessage = "Loading...";
+  statusMessage = tr(STR_LOADING);
   entries.clear();
   selectorIndex = 0;
   requestUpdate();
@@ -293,7 +294,7 @@ void OpdsBookBrowserActivity::navigateBack() {
     navigationHistory.pop_back();
 
     state = BrowserState::LOADING;
-    statusMessage = "Loading...";
+    statusMessage = tr(STR_LOADING);
     entries.clear();
     selectorIndex = 0;
     requestUpdate();
@@ -340,7 +341,7 @@ void OpdsBookBrowserActivity::downloadBook(const OpdsEntry& book) {
     requestUpdate();
   } else {
     state = BrowserState::ERROR;
-    errorMessage = "Download failed";
+    errorMessage = tr(STR_DOWNLOAD_FAILED);
     requestUpdate();
   }
 }
@@ -349,7 +350,7 @@ void OpdsBookBrowserActivity::checkAndConnectWifi() {
   // Already connected? Verify connection is valid by checking IP
   if (WiFi.status() == WL_CONNECTED && WiFi.localIP() != IPAddress(0, 0, 0, 0)) {
     state = BrowserState::LOADING;
-    statusMessage = "Loading...";
+    statusMessage = tr(STR_LOADING);
     requestUpdate();
     fetchFeed(currentPath);
     return;
@@ -373,7 +374,7 @@ void OpdsBookBrowserActivity::onWifiSelectionComplete(const bool connected) {
   if (connected) {
     LOG_DBG("OPDS", "WiFi connected via selection, fetching feed");
     state = BrowserState::LOADING;
-    statusMessage = "Loading...";
+    statusMessage = tr(STR_LOADING);
     requestUpdate();
     fetchFeed(currentPath);
   } else {
@@ -383,7 +384,7 @@ void OpdsBookBrowserActivity::onWifiSelectionComplete(const bool connected) {
     WiFi.disconnect();
     WiFi.mode(WIFI_OFF);
     state = BrowserState::ERROR;
-    errorMessage = "WiFi connection failed";
+    errorMessage = tr(STR_WIFI_CONN_FAILED);
     requestUpdate();
   }
 }