ci(i18n): detect unused & missing translation keys + clean en.json key debt

Rebased onto develop (post #1663 sentence-case pass). Adds a dependency-free
Bun checker (bun run i18n:check) wired into the lint matrix: flags keys used in
code but missing from en.json, and keys present in en.json but referenced
nowhere. Source-only - only en.json is checked/edited; Crowdin syncs the other
locales.

en.json: +8 missing source keys, -86 dead keys. Planned-feature keys kept and
allow-listed with tracking issues:
- home.settings.other.show_large_home_carousel (#1702 native media-bar)
- home.settings.logs.delete_all_logs (#1703 iOS logs fix)
- home.suggested_episodes (#1704)

login: point the TV password modal at the existing login.login_button instead
of a redundant login.login alias.

.github/workflows/linting.yml

@@ -97,6 +97,7 @@ jobs:
           - "check"
           - "format"
           - "typecheck"
+          - "i18n:check"
 
     steps:
       - name: "📥 Checkout PR code"