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/index.css

@@ -5,6 +5,11 @@
 :root {
   --font-scale: 1.06;
 
+  /* Vertical space the floating top header needs to clear (top offset 12px + pill height 44px +
+     bottom breathing room ~12px). Module content pads its top by this so nothing hides under the
+     header; kept in one place since App and Playlists both reference it. */
+  --hdr-h: 68px;
+
   /* ===== Glass tunables — every value that materially drives the look lives here so it can be
      tuned in ONE place. ===== */
   /* GLOBAL baseline = "Adaptive": solid-enough dark glass that reads well on content-less chrome