fix(desktop): language switching + window drag hardening

[hqhq1025]

Summary

• Bug 1 — Language switching broke in Settings (Appearance tab): AppearanceTab.handleLocaleChange persisted the locale via window.codesign.locale.set() but never called setLocale() from @open-codesign/i18n, so i18next.changeLanguage() was never invoked. The UI only reflected the new language after a full app restart. Fixed by mirroring the correct two-step flow already used by LanguageToggle: persist → applyLocale(persisted) → update component state. Also updated the in-UI hint from "Restart the app after changing the language." to "Language changes take effect immediately."

• Bug 2 — Window drag hardening: TopBar already has WebkitAppRegion: 'drag' on the <header> and 'no-drag' on the button cluster. Added an explicit 'no-drag' inline style directly on the <button> element inside LanguageToggle as a defensive measure against CSS inheritance ambiguity in Electron's hiddenInset mode.

• New test: Added a vitest case to packages/i18n/src/i18n.test.ts that asserts setLocale() immediately updates getCurrentLocale() and i18n.t() without requiring a page reload. Runs clean with the 8 pre-existing tests (9 total now).

Test plan

• pnpm dev → Settings → Appearance → switch zh-CN / en → UI updates immediately (no restart)
• Restart the app → language choice is persisted from previous session
• TopBar LanguageToggle button still works (clickable, not accidentally treated as drag surface)
• macOS: drag the empty space in the TopBar between the wordmark and the button cluster → window moves
• pnpm -r typecheck and pnpm lint both pass
• pnpm exec turbo run test → all 10 packages pass (41 tests total)

Compatibility, upgradeability, no bloat, elegance

• Compatibility ✅ — no schema or IPC changes
• Upgradeability ✅ — no new migration paths
• No bloat ✅ — zero new dependencies; re-uses existing @open-codesign/i18n export
• Elegance ✅ — Settings now follows the same pattern as LanguageToggle

[github-actions]

Findings

• No high-confidence issues found on added/modified lines.

Summary

• Review mode: initial
• No issues to report in the current diff.
• Not found in repo/docs: docs/VISION.md, docs/PRINCIPLES.md.
• Residual risk: no UI/E2E coverage in this PR for the language toggle drag-region behavior or locale-persist failure path in renderer interactions.

Testing

• Not run (automation)

open-codesign Bot