feat(random-mix): playlist size selector + filter panel layout cleanup (#445)

* feat(random-mix): playlist size selector + filter panel layout cleanup

Adds a 5-button playlist-size picker (50/75/100/125/150) at the top of
the Random Mix filter panel, persisted via authStore. Clicking a size
immediately reruns the current mix (genre-scoped or All Songs) at the
new size — no second click on Remix needed.

Filter panel layout cleaned up:

- Two sub-sections "MIX SETTINGS" and "EXCLUSIONS" with a divider
  between them so the panel reads cleanly with the new size row.
- Larger panel-level headers (FILTERS / GENRE MIX) so the hierarchy
  panel-title > sub-section is visually unambiguous.
- Italic muted note under MIX SETTINGS calling out that large mix
  sizes may return fewer unique tracks if the server's random pool
  runs short — sets honest expectations instead of users wondering
  why a 150 request returned ~126.

fetchRandomMixSongsUntilFull now scales batch size, max-batch ceiling
and dup-streak budget with target size; when no Settings-level mix
filter is active, the first call asks for the full target so a 150
mix can finish in a single round-trip on most libraries. The loop
falls through to top up with deduped follow-up calls if the server
returns fewer than requested.

* docs(changelog): add #445 Random Mix playlist size selector entry

* chore(credits): add #445 to Psychotoxical contributions

src/pages/Settings.tsx

@@ -351,6 +351,7 @@ const CONTRIBUTORS = [
       'Audio: frame-align gapless-off track-separation silence (fixes mono-channel playback after natural track end) (PR #439)',
       'Settings: 3-state animation mode (Full / Reduced / Static) — replaces boolean reduce-animations toggle (PR #441)',
       'Tracks: Highly Rated rail and per-card star display, with cache layer for ndListSongs (PR #443)',
+      'Random Mix: playlist-size picker (50/75/100/125/150) and filter-panel layout cleanup (PR #445)',
     ],
   },
 ] as const;