feat: add i18n support with VITE_LOCALE env var (en/fr)

- Lightweight i18n system in src/lib/i18n.ts (no external deps)
- All UI strings extracted to translation keys
- English (default) and French locales included
- Set VITE_LOCALE=fr in .env for French UI
- Fallback to English for unknown locales

src/lib/i18n.ts

@@ -0,0 +1,93 @@
+/**
+ * Lightweight i18n — no external deps.
+ * Locale is set via VITE_LOCALE env var (default: "en").
+ * Add new languages by adding a record to `messages`.
+ */
+
+const en = {
+  // Login screen
+  'login.title': 'PinchChat',
+  'login.subtitle': 'Connect to your OpenClaw gateway',
+  'login.gatewayUrl': 'Gateway URL',
+  'login.token': 'Token',
+  'login.tokenPlaceholder': 'Enter your gateway token',
+  'login.connect': 'Connect',
+  'login.connecting': 'Connecting…',
+  'login.showToken': 'Show token',
+  'login.hideToken': 'Hide token',
+  'login.storedLocally': 'Credentials are stored locally in your browser',
+
+  // Header
+  'header.title': 'PinchChat',
+  'header.connected': 'Connected',
+  'header.disconnected': 'Disconnected',
+  'header.logout': 'Logout',
+  'header.toggleSidebar': 'Toggle sidebar',
+
+  // Chat
+  'chat.welcome': 'PinchChat',
+  'chat.welcomeSub': 'Send a message to get started',
+  'chat.inputPlaceholder': 'Type a message…',
+  'chat.inputLabel': 'Message',
+  'chat.attachFile': 'Attach file',
+  'chat.send': 'Send',
+  'chat.stop': 'Stop',
+  'chat.messages': 'Chat messages',
+
+  // Sidebar
+  'sidebar.title': 'Sessions',
+  'sidebar.empty': 'No sessions',
+
+  // Thinking
+  'thinking.label': 'Thinking',
+
+  // Tool call
+  'tool.result': 'Result',
+} as const;
+
+const fr: Record<keyof typeof en, string> = {
+  'login.title': 'PinchChat',
+  'login.subtitle': 'Connectez-vous à votre gateway OpenClaw',
+  'login.gatewayUrl': 'URL de la gateway',
+  'login.token': 'Token',
+  'login.tokenPlaceholder': 'Entrez votre token gateway',
+  'login.connect': 'Connexion',
+  'login.connecting': 'Connexion…',
+  'login.showToken': 'Afficher le token',
+  'login.hideToken': 'Masquer le token',
+  'login.storedLocally': 'Les identifiants sont stockés localement dans votre navigateur',
+
+  'header.title': 'PinchChat',
+  'header.connected': 'Connecté',
+  'header.disconnected': 'Déconnecté',
+  'header.logout': 'Déconnexion',
+  'header.toggleSidebar': 'Afficher/masquer la barre latérale',
+
+  'chat.welcome': 'PinchChat',
+  'chat.welcomeSub': 'Envoyez un message pour commencer',
+  'chat.inputPlaceholder': 'Tapez un message…',
+  'chat.inputLabel': 'Message',
+  'chat.attachFile': 'Joindre un fichier',
+  'chat.send': 'Envoyer',
+  'chat.stop': 'Arrêter',
+  'chat.messages': 'Messages du chat',
+
+  'sidebar.title': 'Sessions',
+  'sidebar.empty': 'Aucune session',
+
+  'thinking.label': 'Réflexion',
+
+  'tool.result': 'Résultat',
+};
+
+const messages: Record<string, Record<string, string>> = { en, fr };
+
+const locale = (import.meta.env.VITE_LOCALE as string) || 'en';
+const dict = messages[locale] || messages.en;
+
+/** Return the translated string for the given key, falling back to English. */
+export function t(key: keyof typeof en): string {
+  return dict[key] ?? (messages.en as Record<string, string>)[key] ?? key;
+}
+
+export { locale };