feat: add Traditional Chinese UI and persistent language switching

[dada878]

Adds English/Traditional Chinese language switching to the dashboard, authentication screens, workspace invitations, and shared campaign reports. English remains the default; a validated cookie lets the server render the saved preference on subsequent visits. Switching languages preserves unsaved campaign fields and never translates outgoing messages or stored API values.

The implementation uses a typed, centralized catalog and React context without new dependencies. Providers stay below the root layout so marketing and template pages remain static. Includes contribution documentation and 11 locale tests.

Validation

• npm run typecheck
• npm run lint (no warnings)
• npm test with local TEST_DATABASE_URL: 27 files, 296 tests passed, including Postgres integration tests
• npm run build
• Browser checks: language persistence after reload, unsaved editor values and selected post, mobile sidebar at 390px, localized metadata/report dates, and English marketing pages

Marketing and legal copy, template playbooks, authentication emails, and raw provider errors remain English. Existing campaign defaults and customer-authored content remain unchanged.

[vercel]

@dada878 is attempting to deploy a commit to the diwenne's projects Team on Vercel.

A member of the Team first needs to authorize it.

[diwenne]

Reviewed by Hermes Agent.

Verified locally (not just PR claims):

• tsc --noEmit: clean
• eslint: no warnings
• vitest (i18n suite): 11/11 passed
• next build: succeeds; marketing/template pages confirmed static, dashboard/auth routes confirmed dynamic as described

Scope is clean — t() only ever wraps static string literals, never user-supplied content used as a translation key. Cookie is httpOnly/sameSite=lax/secure-in-prod with server-side validation and English fallback. No schema changes, no new deps. Vercel check failure is just an auth-to-deploy gate, not a build/test failure.

LGTM, merging.