fix(desktop): a late startup locale read never overrides the user's in-session pick

With the startup config read now retried (previous commits, #76158), a retry
can resolve after the user has already chosen a language in Settings and
would repaint the stale on-disk value over it. Mark the explicit pick in a
ref and let every retry outcome (success or exhausted fallback) defer to it.

Idea and guard from #105470 (webtecnica), folded onto the #76158 base.

Co-authored-by: webtecnica <webtecnica@gmail.com>

apps/desktop/src/i18n/context.test.tsx

@@ -310,4 +310,41 @@ describe('I18nProvider', () => {
 
     vi.useRealTimers()
   })
+
+  it('a late startup read never overrides a language the user picked mid-retry', async () => {
+    vi.useFakeTimers()
+
+    const getConfig = vi
+      .fn()
+      .mockRejectedValueOnce(new Error('backend not ready yet'))
+      .mockResolvedValue({ display: { language: 'en' } })
+
+    const configClient: I18nConfigClient = {
+      getConfig,
+      saveConfig: vi.fn().mockResolvedValue({ ok: true })
+    }
+
+    render(
+      <I18nProvider configClient={configClient}>
+        <LanguageProbe target="ja" />
+      </I18nProvider>
+    )
+
+    await act(async () => {})
+    expect(screen.getByTestId('locale').textContent).toBe('en')
+
+    // User picks Japanese while the startup retry is still pending.
+    await act(async () => {
+      fireEvent.click(screen.getByRole('button', { name: 'switch' }))
+    })
+    expect(screen.getByTestId('locale').textContent).toBe('ja')
+
+    // The retry resolves with the stale on-disk value; the explicit pick wins.
+    await act(async () => {
+      vi.advanceTimersByTime(3_000)
+    })
+    expect(screen.getByTestId('locale').textContent).toBe('ja')
+
+    vi.useRealTimers()
+  })
 })

apps/desktop/src/i18n/context.tsx

@@ -99,6 +99,9 @@ export function I18nProvider({ children, configClient = defaultConfigClient, ini
   const [configLoadError, setConfigLoadError] = useState<Error | null>(null)
   const [saveError, setSaveError] = useState<Error | null>(null)
   const localeRef = useRef(locale)
+  // Set once the user picks a language through setLocale: a startup read that
+  // resolves (or fails) after that must never overwrite an explicit choice.
+  const userLocaleRef = useRef(false)
 
   // eslint-disable-next-line no-restricted-syntax -- legitimate non-atom ref write (see eslint rule comment)
   useEffect(() => {
@@ -132,12 +135,12 @@ export function I18nProvider({ children, configClient = defaultConfigClient, ini
       return configClient
         .getConfig()
         .then(config => {
-          if (!cancelled) {
+          if (!cancelled && !userLocaleRef.current) {
             setLocaleState(normalizeLocale(getConfigDisplayLanguage(config)))
           }
         })
         .catch(error => {
-          if (cancelled) {
+          if (cancelled || userLocaleRef.current) {
             return
           }
 
@@ -173,6 +176,7 @@ export function I18nProvider({ children, configClient = defaultConfigClient, ini
     async (next: Locale) => {
       const previousLocale = localeRef.current
 
+      userLocaleRef.current = true
       setSaveError(null)
       setLocaleState(next)