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/settings/ClearCacheActivity.cpp

@@ -2,6 +2,7 @@
 
 #include <GfxRenderer.h>
 #include <HalStorage.h>
+#include <I18n.h>
 #include <Logging.h>
 
 #include "MappedInputManager.h"
@@ -21,46 +22,47 @@ void ClearCacheActivity::render(Activity::RenderLock&&) {
   const auto pageHeight = renderer.getScreenHeight();
 
   renderer.clearScreen();
-  renderer.drawCenteredText(UI_12_FONT_ID, 15, "Clear Cache", true, EpdFontFamily::BOLD);
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, tr(STR_CLEAR_READING_CACHE), true, EpdFontFamily::BOLD);
 
   if (state == WARNING) {
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 60, "This will clear all cached book data.", true);
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 30, "All reading progress will be lost!", true,
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 60, tr(STR_CLEAR_CACHE_WARNING_1), true);
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 30, tr(STR_CLEAR_CACHE_WARNING_2), true,
                               EpdFontFamily::BOLD);
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 10, "Books will need to be re-indexed", true);
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 30, "when opened again.", true);
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 10, tr(STR_CLEAR_CACHE_WARNING_3), true);
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 30, tr(STR_CLEAR_CACHE_WARNING_4), true);
 
-    const auto labels = mappedInput.mapLabels("« Cancel", "Clear", "", "");
+    const auto labels = mappedInput.mapLabels(tr(STR_CANCEL), tr(STR_CLEAR_BUTTON), "", "");
     GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
     renderer.displayBuffer();
     return;
   }
 
   if (state == CLEARING) {
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, "Clearing cache...", true, EpdFontFamily::BOLD);
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2, tr(STR_CLEARING_CACHE), true, EpdFontFamily::BOLD);
     renderer.displayBuffer();
     return;
   }
 
   if (state == SUCCESS) {
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 20, "Cache Cleared", true, EpdFontFamily::BOLD);
-    String resultText = String(clearedCount) + " items removed";
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 20, tr(STR_CACHE_CLEARED), true, EpdFontFamily::BOLD);
+    std::string resultText = std::to_string(clearedCount) + " " + std::string(tr(STR_ITEMS_REMOVED));
     if (failedCount > 0) {
-      resultText += ", " + String(failedCount) + " failed";
+      resultText += ", " + std::to_string(failedCount) + " " + std::string(tr(STR_FAILED_LOWER));
     }
     renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 10, resultText.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 == FAILED) {
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 20, "Failed to clear cache", true, EpdFontFamily::BOLD);
-    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 10, "Check serial output for details");
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 - 20, tr(STR_CLEAR_CACHE_FAILED), true,
+                              EpdFontFamily::BOLD);
+    renderer.drawCenteredText(UI_10_FONT_ID, pageHeight / 2 + 10, tr(STR_CHECK_SERIAL_OUTPUT));
 
-    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;