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/locales/en.ts

@@ -325,6 +325,10 @@ export const enTranslation = {
     favoriteRemove: 'Remove from Favorites',
     play: 'Play',
     trackGenre: 'Genre',
+    mixSettingsHeader: 'Mix Settings',
+    exclusionsHeader: 'Exclusions',
+    filterPanelInexactSizeNote: 'Mix size is a target — large requests may return fewer unique tracks if the server\'s random pool runs short.',
+    mixSize: 'Playlist size',
     excludeAudiobooks: 'Exclude audiobooks & radio plays',
     excludeAudiobooksDesc: 'Matches keywords against genre, title, album, and artist — e.g. Hörbuch, Audiobook, Spoken Word, …',
     genreBlocked: 'Keyword blocked',