fix: route legacy shared_preferences writes through the desktop store

[ianneub]

Fixes #3286

Problem
DesktopSharedPreferencesStore replaced only the async prefs backend. The legacy SharedPreferences API, which emoji_picker_flutter still uses to save recent emojis, went to the stock Linux/Windows backend. That backend rewrites the whole shared shared_preferences.json from a snapshot taken when the isolate started. One emoji pick could revert or wipe every setting written since then, including serverAddress and finishedSetup. On the next launch the app then cleared the database and returned to setup.

Solution
DesktopSharedPreferencesStore.register() now also installs _DesktopLegacySharedPreferencesStore, a small SharedPreferencesStorePlatform adapter that reuses the store's existing _readFile/_mutate. Legacy calls re-read the file, and legacy writes go through the existing lock + atomic write + .bak path. Legacy keys stay as they are (flutter.-prefixed, same file), so the legacy→async migration and existing data are unaffected. _mutate/_atomicWrite now report whether the file was written, so legacy writes return false on failure like the stock backends. Also updated settings/CLAUDE.md.

Testing (Linux, Flutter 3.44.6)

• Throwaway flutter test repro (not committed; the repo has no test suite). It seeds the prefs file, calls register() and SharedPreferences.getInstance(), writes lastOpenedChat via SharedPreferencesAsync, then calls legacy.setString('recent', '[]'). On this branch it also checks remove, setStringList, clear, the deprecated *WithPrefix methods, and that a forced write failure returns false without touching the file:
  • master @ e2eaced: file ends as [serverAddress, finishedSetup, flutter.recent], so lastOpenedChat is lost ❌
  • this branch: [serverAddress, finishedSetup, lastOpenedChat, flutter.recent] ✅
• flutter build linux --release, run against a copy of a real profile (146 keys) and a real server. The app starts with setup intact and syncs. After opening a chat and picking an emoji, flutter.recent updated, lastOpenedChat kept its new value, and no keys were lost.
• flutter analyze on lib/services/backend/settings/: no issues.
• Not tested on Windows. It uses the same registration path.

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https://claude.ai/code/session_01AyxJz1fsojfXbxJUBBsivy