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/network/NetworkModeSelectionActivity.cpp

@@ -1,6 +1,7 @@
 #include "NetworkModeSelectionActivity.h"
 
 #include <GfxRenderer.h>
+#include <I18n.h>
 
 #include "MappedInputManager.h"
 #include "components/UITheme.h"
@@ -8,12 +9,6 @@
 
 namespace {
 constexpr int MENU_ITEM_COUNT = 3;
-const char* MENU_ITEMS[MENU_ITEM_COUNT] = {"Join a Network", "Connect to Calibre", "Create Hotspot"};
-const char* MENU_DESCRIPTIONS[MENU_ITEM_COUNT] = {
-    "Connect to an existing WiFi network",
-    "Use Calibre wireless device transfers",
-    "Create a WiFi network others can join",
-};
 }  // namespace
 
 void NetworkModeSelectionActivity::onEnter() {
@@ -66,10 +61,16 @@ void NetworkModeSelectionActivity::render(Activity::RenderLock&&) {
   const auto pageHeight = renderer.getScreenHeight();
 
   // Draw header
-  renderer.drawCenteredText(UI_12_FONT_ID, 15, "File Transfer", true, EpdFontFamily::BOLD);
+  renderer.drawCenteredText(UI_12_FONT_ID, 15, tr(STR_FILE_TRANSFER), true, EpdFontFamily::BOLD);
 
   // Draw subtitle
-  renderer.drawCenteredText(UI_10_FONT_ID, 50, "How would you like to connect?");
+  renderer.drawCenteredText(UI_10_FONT_ID, 50, tr(STR_HOW_CONNECT));
+
+  // Menu items and descriptions
+  static constexpr StrId menuItems[MENU_ITEM_COUNT] = {StrId::STR_JOIN_NETWORK, StrId::STR_CALIBRE_WIRELESS,
+                                                       StrId::STR_CREATE_HOTSPOT};
+  static constexpr StrId menuDescs[MENU_ITEM_COUNT] = {StrId::STR_JOIN_DESC, StrId::STR_CALIBRE_DESC,
+                                                       StrId::STR_HOTSPOT_DESC};
 
   // Draw menu items centered on screen
   constexpr int itemHeight = 50;  // Height for each menu item (including description)
@@ -86,12 +87,12 @@ void NetworkModeSelectionActivity::render(Activity::RenderLock&&) {
 
     // Draw text: black=false (white text) when selected (on black background)
     //            black=true (black text) when not selected (on white background)
-    renderer.drawText(UI_10_FONT_ID, 30, itemY, MENU_ITEMS[i], /*black=*/!isSelected);
-    renderer.drawText(SMALL_FONT_ID, 30, itemY + 22, MENU_DESCRIPTIONS[i], /*black=*/!isSelected);
+    renderer.drawText(UI_10_FONT_ID, 30, itemY, I18N.get(menuItems[i]), /*black=*/!isSelected);
+    renderer.drawText(SMALL_FONT_ID, 30, itemY + 22, I18N.get(menuDescs[i]), /*black=*/!isSelected);
   }
 
   // Draw help text at bottom
-  const auto labels = mappedInput.mapLabels("« Back", "Select", "", "");
+  const auto labels = mappedInput.mapLabels(tr(STR_BACK), tr(STR_SELECT), "", "");
   GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
 
   renderer.displayBuffer();