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.

scripts/i18n-keys.config.json

@@ -0,0 +1,46 @@
+{
+  "localesDir": "translations",
+  "sourceLocale": "en",
+  "srcDirs": [
+    "app",
+    "components",
+    "hooks",
+    "providers",
+    "utils",
+    "modules",
+    "packages",
+    "constants"
+  ],
+  "srcExtensions": [".ts", ".tsx", ".js", ".jsx", ".mjs", ".cjs"],
+  "excludeDirs": [
+    "node_modules",
+    "ios",
+    "android",
+    ".expo",
+    ".git",
+    "dist",
+    "build",
+    "translations"
+  ],
+  "_ignoreUnusedNote": "Keys for planned features that are intentionally kept in en.json but not yet wired in code. They are exempt from the unused-key check until implemented. Remove an entry once its feature ships and uses the key. Pending implementation: home.settings.other.show_large_home_carousel (#1702 media-bar), home.settings.logs.delete_all_logs (#1703 iOS logs fix), home.suggested_episodes (#1704).",
+  "ignoreUnused": [
+    "watchlists.add_to_watchlist",
+    "watchlists.remove_from_watchlist",
+    "watchlists.create_one_first",
+    "watchlists.no_compatible_watchlists",
+    "pin.confirm_pin",
+    "pin.pins_dont_match",
+    "player.search_subtitles",
+    "player.subtitle_search",
+    "player.subtitle_download_hint",
+    "player.subtitle_tracks",
+    "player.using_jellyfin_server",
+    "player.swipe_down_settings",
+    "player.stopPlayback",
+    "player.stopPlayingTitle",
+    "player.stopPlayingConfirm",
+    "home.settings.other.show_large_home_carousel",
+    "home.settings.logs.delete_all_logs",
+    "home.suggested_episodes"
+  ]
+}