feat(app): add Chinese (Simplified) as a 7th UI language

Translate every key in Loc.Strings and register "zh" in Loc.Languages
/LanguageNames. Wire it into the Settings language picker, which is a
hand-written RadioButton list (not generated from Loc.Languages), so
add IsLangZh to MainWindowViewModel and the matching button in
MainView.axaml. Update CLAUDE.md/AGENTS.md language count and note the
non-dynamic picker to avoid the same gap next time.

AGENTS.md

@@ -92,7 +92,7 @@ docker/           reproducible release builds (build.sh + pinned Dockerfiles)
 - **In-app overlays, not OS windows:** details (address, transaction), settings, and help are full-screen `Border`s gated by an `IsXxxOpen` flag, not separate `Window`s — instant open/close, mobile-friendly, and popups/top-levels are slow on WSLg.
   - Pattern: bool property + Open/Close commands + backdrop handler and Esc key in `MainView`'s code-behind; overlay close buttons bind via `$parent[UserControl]` (not `$parent[Window]`, absent on mobile).
   - Heavy network work runs off the UI thread (`Task.Run`) so the overlay never freezes.
-- **Localization:** `Localization/Loc.cs`, key→6 languages dictionary (it/en/es/fr/pt/de); in XAML `{Binding Loc[key]}`, in C# `Loc.Tr("key")`. On language change the `Loc` instance is replaced.
+- **Localization:** `Localization/Loc.cs`, key→7 languages dictionary (it/en/es/fr/pt/de/zh); in XAML `{Binding Loc[key]}`, in C# `Loc.Tr("key")`. On language change the `Loc` instance is replaced. The language picker in the Settings overlay (`MainView.axaml`) is a hand-written list of `RadioButton`s bound to `IsLangXx` properties in `MainWindowViewModel.Settings.cs` — it does **not** read `Loc.Languages` dynamically, so a new language needs a button + property added there too, not just a dictionary column.
 - **App version:** single source is `<Version>` in `src/App/PalladiumWallet.App.csproj`, read at runtime (`MainWindowViewModel.AppVersion`) and shown in the title. `Core/Net/UpdateChecker.cs` compares it against the latest GitHub release on startup; `MainWindowViewModel.Update.cs` prompts the user if newer.
 - **Storage paths:** `Core/Storage/AppPaths` resolves data locations; `AppPaths.OverrideDataRoot` (top priority) is the per-platform seam — the Android head sets it to the app sandbox (`Context.FilesDir`), desktop leaves it null.