fix(ui): consolidate Orbit / Server / Live header dropdowns (#725)

* fix(ui): consolidate Orbit / Server / Live header dropdowns

The three header dropdown popovers each had their own container style.
Live used a glass utility class with backdrop-filter that read poorly on
many themes (reported by cucadmuh). Move all three onto the shared
`.nav-library-dropdown-panel` container — same bg, border, shadow and
radius via existing semantic tokens (`--bg-card`, `--border-dropdown`,
`--shadow-dropdown`, `--radius-md`). Item styles per dropdown stay
case-specific.

- NowPlayingDropdown: replace `glass animate-fade-in` with
  `nav-library-dropdown-panel animate-fade-in`; drop now-redundant inline
  borderRadius / boxShadow / zIndex; keep padding + gap for the
  user-list breathing room.
- OrbitStartTrigger: add `nav-library-dropdown-panel` alongside
  `orbit-launch-pop`. The component-local class is now reduced to the
  menu-specific min-width + tighter item gap; bg / border / shadow /
  radius / padding inherit from the shared container.
- Drop the unused `.glass` utility class; no other call sites.

* docs(changelog): header dropdown consistency fix (#725)

src/styles/themes/utility-classes.css

@@ -1,11 +1,4 @@
 /* ─── Utility Classes ─── */
-.glass {
-  background: var(--bg-glass, rgba(30, 30, 46, 0.75));
-  backdrop-filter: blur(20px);
-  -webkit-backdrop-filter: blur(20px);
-  border: 1px solid var(--border-subtle);
-}
-
 .gradient-text {
   background: linear-gradient(135deg, var(--ctp-mauve), var(--ctp-blue));
   -webkit-background-clip: text;