feat(login): add language dropdown, browser auto-detection and configurable default

Replace the language cycling button on the login page with a dropdown
showing all 14 supported languages. Add automatic browser/OS language
detection via navigator.languages, falling back to a configurable
DEFAULT_LANGUAGE env var, then 'en' as last resort.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/store/settingsStore.ts

@@ -10,6 +10,7 @@ interface SettingsState {
   loadSettings: () => Promise<void>
   updateSetting: (key: keyof Settings, value: Settings[keyof Settings]) => Promise<void>
   setLanguageLocal: (lang: string) => void
+  setLanguageTransient: (lang: string) => void
   updateSettings: (settingsObj: Partial<Settings>) => Promise<void>
 }
 
@@ -59,6 +60,13 @@ export const useSettingsStore = create<SettingsState>((set, get) => ({
     set((state) => ({ settings: { ...state.settings, language: lang } }))
   },
 
+  // Applies a language for the current session without persisting to localStorage.
+  // Used for automatic detection (browser/server default) — only explicit user
+  // choices via the UI should be persisted.
+  setLanguageTransient: (lang: string) => {
+    set((state) => ({ settings: { ...state.settings, language: lang } }))
+  },
+
   updateSettings: async (settingsObj: Partial<Settings>) => {
     set((state) => ({
       settings: { ...state.settings, ...settingsObj },