fix: persist ui language across browser reloads

[rezawr]

Fixes #828 — UI language choice was not being persisted across browser reloads.

Changes

• src/mock-tauri/mock-handlers.ts — hydrate the mock Settings from localStorage on module load, and write through to localStorage on every save_settings call. try/catch guards so a private-mode / quota failure degrades to in-memory only instead of throwing.
• src/components/SettingsPanel.tsx + SettingsFooter.tsx — handleSave is now async: it awaits the onSave promise, then closes the panel. The Close and Save buttons are disabled while a save is in flight, so the panel can't be dismissed mid-write.
• src/mock-tauri/mock-handlers.coverage.test.ts — new regression test: writes ui_language: 'it-IT', reloads the handlers module, asserts the language is still 'it-IT'.
• src/mock-tauri/mock-handlers.more.test.ts — localStorage.clear() in beforeEach so the new persistence doesn't leak between tests.
• src/components/SettingsPanel.test.tsx — new test pinning the async-save contract: onClose is not called until the save promise resolves, and the Save button is disabled in the meantime.