fix(i18n): translate the player, downloads and server-setup text left in English

A Portuguese user reported "Skip Intro" rendering in English on Android TV.
The locale files were not the problem - all 22 were structurally complete.
skip_marker_button.dart simply never imported strings.g.dart and assigned
'Skip Intro' / 'Skip Credits' / 'Next Episode' as plain literals. An audit of
lib/ found ~120 more sites in the same state, in four shapes that need
different fixes:

A literal in a file that never imported the i18n layer is the easy one -
skip_marker_button, performance_stats, track_label_builder and codec_utils all
render text with no `t` in the file at all. TrackLabelBuilder._compose now takes
a fallbackLabel builder instead of an English fallbackPrefix, so the caller
supplies t.audioTracks.track / t.videoControls.subtitleTrack and every unnamed
audio and subtitle row in the track menus is localized.

English reaching the user through an exception message is the widest one, and
it needs care: MediaServerException.message feeds both toString() - logs and
Sentry grouping - and verbatim UI display. Localizing it in place would make
bug-report logs follow the user's locale and split one Sentry issue into 22.
The MediaServer and Seerr families instead gain a nullable `display` alongside
the English `message`, and the six screens that print these errors read
`display ?? message`. PlaybackException keeps the opposite rule, because it
already carries a PlaybackFailureReason for logic and classifyPlaybackFailure
already builds it from t.messages: its stragglers are localized at the throw
site. That also removes the literal "Exception: " prefix Live TV users saw on
a tune failure, since PlaybackException.toString() returns the bare message.

Localized parts hand-concatenated with bare English are the shape no search for
Text('...') can find: '${t.common.pause} auto-scroll' on the home carousel,
'${day} at ${time}' on the Live TV schedule row, and an actor-screen count that
hand-rolled its plural as `n == 1 ? 'title' : 'titles'` - wrong for ru and pl
regardless of translation, now a real Slang plural.

Finally a literal assigned to provider state that a widget renders later:
DownloadProgress.errorMessage, and the four background_downloader notification
bodies, which sit inside a plugin config call where no widget-shaped search
reaches them.

Two things surfaced while converting. track_chapter_controls compared a track
label against 'Audio Track N' to swap in a localized version; once the builder
localized its own fallback that branch became unreachable, so it and the
orphaned _joinTrackLabel are gone. And discovery_view's PeerError fallback arm
looks like a leak but is not - its producers already localize, and a test says
so - so it stays as it is.

All 21 non-base locales are translated, including the 21 keys left empty by
earlier commits that were falling back to English. No locale has an empty value.

scripts/check_hardcoded_strings.py guards the three shapes a structural check
can see, and runs in ci_checks.sh after translation hygiene. Its first draft
passed its own tests while missing this very bug, because 'Skip Intro' is bound
to a local rather than handed to Text(); the name-bound rule that closes that
gap is restricted to phrase-shaped literals, or it cannot tell copy from the
identifiers this codebase binds constantly ('cast_row', 'auto', 'liveTv'). It
cannot see English inside a throw or assigned to a provider field - neither is
distinguishable from a log message without dataflow analysis - and the docstring
says so. label: and actionLabel: are deliberately unscanned: here they name a
diagnostic operation, and a check that is chronically red is a check that gets
switched off.

One commit rather than one per area: the keys, the 22 locale files and the
generated output are a single unit, and any partial split fails the repo's own
unused-key scan on the way through.

close #1856

scripts/hardcoded_strings_allowlist.json

@@ -0,0 +1,85 @@
+{
+  "*": {
+    "Plex": "Brand name",
+    "Jellyfin": "Brand name",
+    "Emby": "Brand name",
+    "Plezy": "Brand name",
+    "Simkl": "Brand name",
+    "BL": "Technical token",
+    "EL": "Technical token",
+    "RPU": "Technical token",
+    "dontkillmyapp.com": "External technical resource name"
+  },
+  "lib/screens/auth_screen.dart": {
+    "Debug: Enter Plex Token": "kDebugMode developer authentication affordance",
+    "Plex Auth Token": "kDebugMode developer authentication affordance",
+    "Enter your Plex.tv token": "kDebugMode developer authentication affordance",
+    "Auth service not ready": "kDebugMode developer authentication diagnostic"
+  },
+  "lib/screens/settings/settings_screen.dart": {
+    "Test Sentry": "kDebugMode developer diagnostic affordance",
+    "Send a test error": "kDebugMode developer diagnostic affordance",
+    "Test ANR": "kDebugMode developer diagnostic affordance",
+    "Block the main thread for 10 seconds": "kDebugMode developer diagnostic affordance",
+    "Blocking main thread...": "kDebugMode developer diagnostic affordance"
+  },
+  "lib/widgets/video_controls/sheets/video_settings_sheet.dart": {
+    "Trigger MPV Fallback": "kDebugMode developer playback affordance",
+    "Simulate HTTP $status from server": "kDebugMode developer playback affordance"
+  },
+  "lib/screens/settings/seerr_connect_screen.dart": {
+    "https://seerr.example.com": "Intentional server URL example"
+  },
+  "lib/screens/settings/external_player_screen.dart": {
+    "myplayer://play?url=": "Intentional URL scheme example",
+    "com.example.player": "Intentional Android package example",
+    "mpv": "Intentional executable example",
+    "/usr/bin/player": "Intentional executable path example"
+  },
+  "lib/screens/settings/appearance_settings_screen.dart": {
+    "English": "Language endonym",
+    "Svenska": "Language endonym",
+    "Français": "Language endonym",
+    "Italiano": "Language endonym",
+    "Nederlands": "Language endonym",
+    "Deutsch": "Language endonym",
+    "Magyar": "Language endonym",
+    "简体中文": "Language endonym",
+    "繁體中文": "Language endonym",
+    "한국어": "Language endonym",
+    "Español": "Language endonym",
+    "Português": "Language endonym",
+    "日本語": "Language endonym",
+    "Русский": "Language endonym",
+    "Polski": "Language endonym",
+    "Dansk": "Language endonym",
+    "Norsk bokmål": "Language endonym",
+    "Български": "Language endonym",
+    "Türkçe": "Language endonym",
+    "Azərbaycanca": "Language endonym",
+    "Қазақша": "Language endonym",
+    "Oʻzbekcha": "Language endonym"
+  },
+  "lib/screens/settings/logs_screen.dart": {
+    "Android ${info.version.release} (API ${info.version.sdkInt})": "English bug-report diagnostic",
+    "TV mode: yes$suffix": "English bug-report diagnostic",
+    "Renderer: $renderer": "English bug-report diagnostic",
+    "Background: ${backgroundWork.describeSync()}": "English bug-report diagnostic",
+    "iOS ${info.systemVersion}": "English bug-report diagnostic",
+    "macOS ${info.osRelease}": "English bug-report diagnostic",
+    "Linux ${info.versionId ?? info.id}": "English bug-report diagnostic",
+    "Effects: ${DevicePerformance.describeSync()}": "English bug-report diagnostic",
+    "Display: ${DevicePerformance.describeDisplay()}": "English bug-report diagnostic"
+  },
+  "lib/services/startup_diagnostics.dart": {
+    "Plezy startup failure": "English bug-report diagnostic",
+    "Version: $appVersion": "English bug-report diagnostic",
+    "Platform: $platform": "English bug-report diagnostic",
+    "When: ${timestamp.toUtc().toIso8601String()}": "English bug-report diagnostic",
+    "Phase: $phaseId": "English bug-report diagnostic",
+    "Error: $errorType": "English bug-report diagnostic",
+    "Repair offered: ${repairable ? 'yes' : 'no'}": "English bug-report diagnostic",
+    "Message: $message": "English bug-report diagnostic",
+    "Stack trace:": "English bug-report diagnostic"
+  }
+}