feat(i18n): add i18n support

.agents/skills/i18n/SKILL.md

@@ -0,0 +1,48 @@
+---
+name: i18n
+description: Implement and review Cursor BYOK desktop localization, including t() usage, locale catalogs, system-language selection, translation completeness, and language settings under apps/desktop.
+---
+
+# Desktop i18n
+
+Keep localization organized as:
+
+```text
+apps/desktop/
+├── plugins/static-i18n-plugin.ts
+└── src/i18n/
+    ├── I18nRoot.tsx
+    ├── runtime.ts
+    ├── store.ts
+    ├── generated/catalog.json
+    └── locales/
+        ├── zh-CN.json
+        └── en-US.json
+```
+
+## Author messages
+
+- Use the global `t()` function with a static Simplified Chinese string literal: `t("保存")`.
+- Do not import or locally declare `t`.
+- Keep calls inside render paths when the result must update after a language change. Do not translate module-level UI constants.
+- Use named placeholders with a static object: `t("第 {page} 页", { page })`.
+- Preserve every placeholder name exactly in all translations.
+- Wrap all user-visible labels, descriptions, tooltips, empty states, validation messages, and accessibility labels. Do not wrap protocol tokens, model IDs, URLs, or product names that should remain unchanged.
+
+## Locale behavior
+
+- `system` is the default preference.
+- Resolve supported operating-system languages to their locale and fall back to `en-US` for every unsupported language.
+- Persist only explicit locale selections. Removing the preference restores system-language behavior.
+- Apply changes immediately and update the document `lang` attribute.
+
+## Update catalogs
+
+From `apps/desktop`:
+
+1. Run `npm run i18n:scan` after adding or changing `t()` sources.
+2. Translate every empty entry in `src/i18n/locales/en-US.json`.
+3. Never edit `src/i18n/generated/catalog.json` or `zh-CN.json` manually; scanning owns them.
+4. Run `npm run check`. A normal build must fail on a missing translation or mismatched placeholder.
+
+When adding another locale, add it to the plugin's supported locales, runtime locale type and messages, locale resolver, settings options, and provide a complete locale JSON file.