fix(settings): the offline language mirror does not outlive the account (#2146)

The mirror of the account language stayed in localStorage past logout, so the next account on the device booted in the previous user's language forever. It is cleared on logout and rewritten on login, and the rate-limit message follows the same order the boot language does.

client/src/store/settingsStore.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest'
-import { DEFAULT_SETTINGS, useSettingsStore } from './settingsStore'
+import { DEFAULT_SETTINGS, forgetServerLanguage, useSettingsStore } from './settingsStore'
 import { settingsApi } from '../api/client'
 import { clearTileCache } from '../sync/tilePrefetcher'
 
@@ -128,4 +128,35 @@ describe('offline language fallback', () => {
     const fresh = await import('./settingsStore')
     expect(fresh.DEFAULT_SETTINGS.language).toBe('de')
   })
+
+  // The account of the user who just signed in decides, and an account without a
+  // language of its own sends no language key at all. Leaving the previous
+  // account's mirror standing is what stranded the next user on a shared browser.
+  it('SETTINGS-LANG-005: an account with no language of its own clears the mirror', async () => {
+    localStorage.setItem('app_language_server', 'ja')
+    vi.mocked(settingsApi.get).mockResolvedValue({ settings: { default_currency: 'EUR' } } as never)
+
+    await useSettingsStore.getState().loadSettings()
+
+    expect(localStorage.getItem('app_language_server')).toBeNull()
+  })
+
+  it('SETTINGS-LANG-006: a language TREK does not ship never reaches the mirror', async () => {
+    localStorage.setItem('app_language_server', 'ja')
+    vi.mocked(settingsApi.get).mockResolvedValue({ settings: { language: 'kl' } } as never)
+
+    await useSettingsStore.getState().loadSettings()
+
+    expect(localStorage.getItem('app_language_server')).toBeNull()
+  })
+
+  it('SETTINGS-LANG-007: forgetServerLanguage drops the mirror and leaves the explicit choice alone', () => {
+    localStorage.setItem('app_language', 'de')
+    localStorage.setItem('app_language_server', 'ja')
+
+    forgetServerLanguage()
+
+    expect(localStorage.getItem('app_language_server')).toBeNull()
+    expect(localStorage.getItem('app_language')).toBe('de')
+  })
 })