feat(i18n): translate remaining components (HU/EN/DE)

Feed, VideoCard, Sidebar, PlayerModal, Channels, Stats, SettingsPanel,
OnboardingWizard, NotificationCenter, Toaster, ErrorBoundary and the relativeTime
helper are now fully translated in Hungarian, English and German, each with its own
locale area file (auto-loaded). Key parity verified across all three languages.

frontend/src/i18n/locales/en/card.json

@@ -0,0 +1,17 @@
+{
+  "watchedUnmark": "Watched — click to unmark",
+  "markWatched": "Mark watched",
+  "savedRemove": "Saved — click to remove",
+  "saveForLater": "Save for later",
+  "unhide": "Unhide",
+  "hide": "Hide",
+  "onlyThisChannel": "Only this channel",
+  "thisChannel": "This channel",
+  "continueTitle": "Continue where you left off",
+  "continue": "Continue",
+  "restartTitle": "Play from the beginning",
+  "restart": "Restart",
+  "play": "Play",
+  "stream": "stream",
+  "views": "views"
+}