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

.env.example

@@ -1,3 +1,6 @@
 # Optional: pre-fill the Gateway URL field on the login screen
 # If not set, defaults to ws://<current-hostname>:18789
 VITE_GATEWAY_WS_URL=ws://localhost:18789
+
+# Optional: UI locale (default: en). Supported: en, fr
+VITE_LOCALE=en