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/LanguageSelectActivity.cpp

@@ -0,0 +1,94 @@
+#include "LanguageSelectActivity.h"
+
+#include <GfxRenderer.h>
+#include <I18n.h>
+
+#include "MappedInputManager.h"
+#include "fontIds.h"
+
+void LanguageSelectActivity::onEnter() {
+  Activity::onEnter();
+
+  totalItems = getLanguageCount();
+
+  // Set current selection based on current language
+  selectedIndex = static_cast<int>(I18N.getLanguage());
+
+  requestUpdate();
+}
+
+void LanguageSelectActivity::onExit() { Activity::onExit(); }
+
+void LanguageSelectActivity::loop() {
+  if (mappedInput.wasPressed(MappedInputManager::Button::Back)) {
+    onBack();
+    return;
+  }
+
+  if (mappedInput.wasPressed(MappedInputManager::Button::Confirm)) {
+    handleSelection();
+    return;
+  }
+
+  if (mappedInput.wasPressed(MappedInputManager::Button::Up) ||
+      mappedInput.wasPressed(MappedInputManager::Button::Left)) {
+    selectedIndex = (selectedIndex + totalItems - 1) % totalItems;
+    requestUpdate();
+  } else if (mappedInput.wasPressed(MappedInputManager::Button::Down) ||
+             mappedInput.wasPressed(MappedInputManager::Button::Right)) {
+    selectedIndex = (selectedIndex + 1) % totalItems;
+    requestUpdate();
+  }
+}
+
+void LanguageSelectActivity::handleSelection() {
+  {
+    RenderLock lock(*this);
+    I18N.setLanguage(static_cast<Language>(selectedIndex));
+  }
+
+  // Return to previous page
+  onBack();
+}
+
+void LanguageSelectActivity::render(Activity::RenderLock&&) {
+  renderer.clearScreen();
+
+  const auto pageWidth = renderer.getScreenWidth();
+  constexpr int rowHeight = 30;
+
+  // Title
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, tr(STR_LANGUAGE), true, EpdFontFamily::BOLD);
+
+  // Current language marker
+  const int currentLang = static_cast<int>(I18N.getLanguage());
+
+  // Draw options
+  for (int i = 0; i < totalItems; i++) {
+    const int itemY = 60 + i * rowHeight;
+    const bool isSelected = (i == selectedIndex);
+    const bool isCurrent = (i == currentLang);
+
+    // Draw selection highlight
+    if (isSelected) {
+      renderer.fillRect(0, itemY - 2, pageWidth - 1, rowHeight);
+    }
+
+    // Draw language name - get it from i18n system
+    const char* langName = I18N.getLanguageName(static_cast<Language>(i));
+    renderer.drawText(UI_10_FONT_ID, 20, itemY, langName, !isSelected);
+
+    // Draw current selection marker
+    if (isCurrent) {
+      const char* marker = tr(STR_ON_MARKER);
+      const auto width = renderer.getTextWidth(UI_10_FONT_ID, marker);
+      renderer.drawText(UI_10_FONT_ID, pageWidth - 20 - width, itemY, marker, !isSelected);
+    }
+  }
+
+  // Button hints
+  const auto labels = mappedInput.mapLabels(tr(STR_BACK), tr(STR_SELECT), "", "");
+  GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
+
+  renderer.displayBuffer();
+}