feat(header): floating module header with cyclic nav and shared search

Replace the full-width top bar with a fixed floating header of glass pills: a
fixed-position ModuleName (accent dot + label + prev/next cyclic module stepper)
and a shared SearchBar. The header's left edge is anchored as if the nav rail
and filter sidebar were both open, so it never shifts on collapse or module
change; the right edge leaves a 10% margin.

- ModuleName label width is measured live from the reachable modules' titles in
  the current language, recomputing on a language or account change.
- Module names come from a shared moduleLabelKey (the same short labels as the
  nav rail); NavSidebar and the header now read from one source (lib/modules.ts).
- The arrows step cyclically through moduleOrder(me) — dynamic, no hard-coded
  targets, wrapping at both ends.
- Search is hoisted into the SearchBar for feed (YouTube search), plex, channels
  (including the Discovery tab) and playlists (new); Go button, Enter triggers it.
- The Playlists left rail now reaches the top; header clearance via --hdr-h.

frontend/src/i18n/locales/de/header.json

@@ -4,6 +4,10 @@
   "clearSearch": "Suche löschen",
   "searchYoutube": "Auf YouTube suchen",
   "searchYoutubeTip": "Live auf YouTube nach diesem Begriff suchen (verbraucht gemeinsames API-Kontingent). Du kannst auch Enter drücken.",
+  "go": "Los",
+  "goTip": "Suchen (oder Enter drücken)",
+  "prevModule": "Vorheriges Modul",
+  "nextModule": "Nächstes Modul",
   "channelManager": "Kanalverwaltung",
   "usageStats": "Nutzung & Statistik",
   "scheduler": "Planer",

frontend/src/i18n/locales/de/playlists.json

@@ -7,6 +7,8 @@
   "ytReadonly": "Von YouTube synchronisiert — Änderungen werden zurücksynchronisiert",
   "noneYet": "Noch keine Wiedergabelisten",
   "newPlaylist": "Neue Liste…",
+  "searchPlaceholder": "Wiedergabelisten suchen…",
+  "noMatches": "Keine passende Wiedergabeliste.",
   "itemCount_one": "{{count}} Video",
   "itemCount_other": "{{count}} Videos",
   "pickOne": "Wähle links eine Liste.",

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

@@ -4,6 +4,10 @@
   "clearSearch": "Clear search",
   "searchYoutube": "Search YouTube",
   "searchYoutubeTip": "Search live on YouTube for this term (uses shared API quota). You can also press Enter.",
+  "go": "Go",
+  "goTip": "Search (or press Enter)",
+  "prevModule": "Previous module",
+  "nextModule": "Next module",
   "channelManager": "Channel manager",
   "usageStats": "Usage & stats",
   "scheduler": "Scheduler",

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

@@ -7,6 +7,8 @@
   "ytReadonly": "Synced from YouTube — edits sync back",
   "noneYet": "No playlists yet",
   "newPlaylist": "New playlist…",
+  "searchPlaceholder": "Search playlists…",
+  "noMatches": "No playlists match.",
   "itemCount_one": "{{count}} video",
   "itemCount_other": "{{count}} videos",
   "pickOne": "Pick a playlist on the left.",

frontend/src/i18n/locales/hu/header.json

@@ -4,6 +4,10 @@
   "clearSearch": "Keresés törlése",
   "searchYoutube": "Keresés a YouTube-on",
   "searchYoutubeTip": "Élő keresés a YouTube-on erre a kifejezésre (a közös API-kvótát fogyasztja). Entert is nyomhatsz.",
+  "go": "Mehet",
+  "goTip": "Keresés (vagy Enter)",
+  "prevModule": "Előző modul",
+  "nextModule": "Következő modul",
   "channelManager": "Csatornakezelő",
   "usageStats": "Használat és statisztika",
   "scheduler": "Ütemező",

frontend/src/i18n/locales/hu/playlists.json

@@ -7,6 +7,8 @@
   "ytReadonly": "YouTube-ról szinkronizálva — a módosítások visszaszinkronizálhatók",
   "noneYet": "Még nincs lejátszási lista",
   "newPlaylist": "Új lista…",
+  "searchPlaceholder": "Listák keresése…",
+  "noMatches": "Nincs találó lista.",
   "itemCount_one": "{{count}} videó",
   "itemCount_other": "{{count}} videó",
   "pickOne": "Válassz egy listát a bal oldalon.",