[FEATURE]: Add i18n dictionary parity regression tests for app/ui locales #50772

On the v2 branch there is no test that guards locale dictionaries against drift. Missing keys are by design (English fallback), but nothing currently catches (a) placeholder mismatches between en and translated dicts, (b) keys added to a locale that don't exist in en, or (c) translations silently disappearing in new commits.

Proposal: add lightweight parity tests for packages/app/src/runtime/i18n and packages/ui/src/i18n that assert:

1. {{param}} placeholder sets match en for every key present in zh/zht
2. locale keys are a subset of en (runtime check)
3. the count of missing keys never rises above a baseline constant (currently app 418, ui 51), so coverage can only improve
4. zh and zht always carry the same key set

This mirrors the parity coverage V1 had under packages/app/src/i18n/parity.test.ts, adapted to the "English fallback by design" model of V2. Happy to open a PR with the tests if this direction sounds right.