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>

lib/I18n/I18n.cpp

@@ -0,0 +1,96 @@
+#include "I18n.h"
+
+#include <HalStorage.h>
+#include <HardwareSerial.h>
+#include <Serialization.h>
+
+#include "I18nStrings.h"
+
+using namespace i18n_strings;
+
+// Settings file path
+static constexpr const char* SETTINGS_FILE = "/.crosspoint/language.bin";
+static constexpr uint8_t SETTINGS_VERSION = 1;
+
+I18n& I18n::getInstance() {
+  static I18n instance;
+  return instance;
+}
+
+const char* I18n::get(StrId id) const {
+  const auto index = static_cast<size_t>(id);
+  if (index >= static_cast<size_t>(StrId::_COUNT)) {
+    return "???";
+  }
+
+  // Use generated helper function - no hardcoded switch needed!
+  const char* const* strings = getStringArray(_language);
+  return strings[index];
+}
+
+void I18n::setLanguage(Language lang) {
+  if (lang >= Language::_COUNT) {
+    return;
+  }
+  _language = lang;
+  saveSettings();
+}
+
+const char* I18n::getLanguageName(Language lang) const {
+  const auto index = static_cast<size_t>(lang);
+  if (index >= static_cast<size_t>(Language::_COUNT)) {
+    return "???";
+  }
+  return LANGUAGE_NAMES[index];
+}
+
+void I18n::saveSettings() {
+  Storage.mkdir("/.crosspoint");
+
+  FsFile file;
+  if (!Storage.openFileForWrite("I18N", SETTINGS_FILE, file)) {
+    Serial.printf("[I18N] Failed to save settings\n");
+    return;
+  }
+
+  serialization::writePod(file, SETTINGS_VERSION);
+  serialization::writePod(file, static_cast<uint8_t>(_language));
+
+  file.close();
+  Serial.printf("[I18N] Settings saved: language=%d\n", static_cast<int>(_language));
+}
+
+void I18n::loadSettings() {
+  FsFile file;
+  if (!Storage.openFileForRead("I18N", SETTINGS_FILE, file)) {
+    Serial.printf("[I18N] No settings file, using default (English)\n");
+    return;
+  }
+
+  uint8_t version;
+  serialization::readPod(file, version);
+  if (version != SETTINGS_VERSION) {
+    Serial.printf("[I18N] Settings version mismatch\n");
+    file.close();
+    return;
+  }
+
+  uint8_t lang;
+  serialization::readPod(file, lang);
+  if (lang < static_cast<size_t>(Language::_COUNT)) {
+    _language = static_cast<Language>(lang);
+    Serial.printf("[I18N] Loaded language: %d\n", static_cast<int>(_language));
+  }
+
+  file.close();
+}
+
+// Generate character set for a specific language
+const char* I18n::getCharacterSet(Language lang) {
+  const auto langIndex = static_cast<size_t>(lang);
+  if (langIndex >= static_cast<size_t>(Language::_COUNT)) {
+    lang = Language::ENGLISH;  // Fallback to first language
+  }
+
+  return CHARACTER_SETS[static_cast<size_t>(lang)];
+}