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/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.",