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/de/sidebar.json

@@ -0,0 +1,65 @@
+{
+  "filters": "Filter",
+  "activeCount": "{{count}} aktiv",
+  "clearAll": "Alle löschen",
+  "resetDefaults": "Auf Standard zurücksetzen",
+  "customize": "Seitenleiste anpassen",
+  "done": "Fertig",
+  "editHint": "Zum Umordnen ziehen · Auge zum Ein-/Ausblenden",
+  "channel": "Kanal",
+  "loading": "Wird geladen…",
+  "thisChannel": "Dieser Kanal",
+  "channelCount": "{{count}} Kanal",
+  "channelCount_other": "{{count}} Kanäle",
+  "dragToReorder": "Zum Umordnen ziehen",
+  "showWidget": "Widget einblenden",
+  "hideWidget": "Widget ausblenden",
+  "expand": "Ausklappen",
+  "collapse": "Einklappen",
+  "any": "Beliebig",
+  "all": "Alle",
+  "tagModeTooltip": "Beliebige vs. alle ausgewählten Tags treffen",
+  "custom": "Benutzerdefiniert",
+  "from": "Von",
+  "to": "Bis",
+  "clearDates": "Daten löschen",
+  "widget": {
+    "show": "Anzeigen",
+    "sort": "Sortierung",
+    "date": "Upload-Datum",
+    "content": "Inhaltstyp",
+    "language": "Sprache",
+    "topic": "Thema"
+  },
+  "show": {
+    "unwatched": "Ungesehen",
+    "in_progress": "Angefangen",
+    "all": "Alle",
+    "watched": "Angesehen",
+    "saved": "Gespeichert",
+    "hidden": "Ausgeblendet"
+  },
+  "sort": {
+    "newest": "Neueste",
+    "oldest": "Älteste",
+    "views": "Meistgesehen",
+    "duration_desc": "Längste",
+    "duration_asc": "Kürzeste",
+    "title": "Name (A–Z)",
+    "subscribers": "Kanal-Abonnenten",
+    "priority": "Kanal-Priorität",
+    "shuffle": "Überrasch mich"
+  },
+  "content": {
+    "normal": "Normal",
+    "shorts": "Shorts",
+    "live": "Live / Demnächst"
+  },
+  "datePreset": {
+    "24h": "24 Std.",
+    "1week": "1 Woche",
+    "1month": "1 Monat",
+    "6months": "6 Monate",
+    "1year": "1 Jahr"
+  }
+}