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/feed.json

@@ -0,0 +1,17 @@
+{
+  "loading": "Loading feed…",
+  "loadError": "Couldn't load the feed.",
+  "emptyTitle": "Your feed is empty",
+  "emptyBody": "Connect your YouTube account to import your subscriptions and build your feed.",
+  "setUp": "Set up my feed",
+  "noMatches": "No videos match these filters.",
+  "videoCount_one": "{{formattedCount}} video",
+  "videoCount_other": "{{formattedCount}} videos",
+  "loadingMore": "Loading more…",
+  "hiddenNamed": "Hidden “{{title}}”",
+  "hidden": "Video hidden",
+  "undo": "Undo",
+  "markedWatchedNamed": "Marked watched “{{title}}”",
+  "markedWatched": "Marked watched",
+  "unwatch": "Unwatch"
+}