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/zh.ts

@@ -321,6 +321,10 @@ export const zhTranslation = {
     favoriteRemove: '从收藏中移除',
     play: '播放',
     trackGenre: '流派',
+    mixSettingsHeader: '混音设置',
+    exclusionsHeader: '排除项',
+    filterPanelInexactSizeNote: '混音大小是一个目标 — 当服务器的随机池耗尽时，较大的请求可能会返回较少的唯一曲目。',
+    mixSize: '列表大小',
     excludeAudiobooks: '排除有声读物和广播剧',
     excludeAudiobooksDesc: '根据流派、标题、专辑和艺术家匹配关键词 — 例如 Hörbuch、Audiobook、Spoken Word 等',
     genreBlocked: '关键词已屏蔽',