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/player.json

@@ -0,0 +1,17 @@
+{
+  "loading": "Loading…",
+  "backToOriginal": "Back to the original video",
+  "back": "Back",
+  "close": "Close",
+  "closeEsc": "Close (Esc)",
+  "description": "Description",
+  "noDescription": "No description.",
+  "channel": "Channel",
+  "views": "{{formattedCount}} views",
+  "stream": "stream",
+  "watched": "Watched",
+  "markWatched": "Mark watched",
+  "watchedUnmark": "Watched — click to unmark",
+  "jumpToTime": "Jump to this time",
+  "playInApp": "Play in the in-app player"
+}