refactor(settings): remove redundant Animations 3-state setting (#495)

* refactor(settings): remove redundant Animations 3-state setting under Seekbar Style

The `animationMode` setting (Full / Reduced / Static) duplicated work
the perf-flag system and OS-level reduced-motion preference already
covered:

- `perfFlags.disableMarqueeScroll` already kills marquee scrolling on
  demand, replacing what `static` mode used to gate.
- The `data-perf-disable-animations` html-level switch already strips
  every `*` animation, replacing what `static` mode used to do globally.
- `@media (prefers-reduced-motion: reduce)` honours the OS setting for
  every user that asked for it via system preferences.
- The 30 fps cap that `reduced` mode applied to the seekbar wave was
  better served by per-feature perf toggles cucadmuh added later.

Removed:
- `AnimationMode` type, `animationMode` field + setter from auth store.
- Settings UI block (3 buttons + hint text) under Appearance > Seekbar
  Style.
- `animationMode === 'static'` short-circuit in WaveformSeek's rAF
  effect; `isReduced` skip-every-other-frame logic; `static`-checks in
  `drawNow` / `needsDirectDraw`.
- `animationMode !== 'static'` guard and `data-anim-mode` attribute in
  MarqueeText.
- `[data-anim-mode="static"]` and `[data-anim-mode="reduced"]` rules in
  layout.css.
- Seven i18n keys (animationMode + 6 variants) across all eight
  locales.

Migration: the persist layer strips `animationMode` (and the legacy
`reducedAnimations` boolean predecessor) so anyone who had `'reduced'`
or `'static'` selected silently lands on the former `'full'` path on
first launch after upgrade. No user-facing prompt — the missing setting
just stops existing.

cucadmuh's PR #472 (FPS overlay), #476 (preview-freeze main seekbar,
sleep-recovery hooks, card-hover removal) and #486 (interpolation
anchor reset on resume) are all preserved untouched — they live in
separate effects / files and were not driven by `animationMode`.

* docs(changelog): add Removed section for animationMode setting (PR #495)

* docs(changelog): refine animationMode removal rationale (drop prefers-reduced-motion overstatement)

src/locales/zh.ts

@@ -961,13 +961,6 @@ export const zhTranslation = {
     sidebarLyricsStyleAppleDesc: '活动行固定在顶部，带有平滑弹簧动画。',
     seekbarStyle: '进度条样式',
     seekbarStyleDesc: '选择播放进度条的外观',
-    animationMode: '动画',
-    animationModeDesc: '在较慢的硬件上以视觉流畅度换取更低的 GPU/CPU 占用。',
-    animationModeFull: '完整',
-    animationModeReduced: '减少',
-    animationModeStatic: '静态',
-    animationModeReducedHint: '进度条波形限制为 30 fps；播放器标题以半速滚动。',
-    animationModeStaticHint: '无波形动画；播放器标题被截断而不滚动。',
     seekbarTruewave: '真实波形',
     seekbarPseudowave: '伪波形',
     seekbarLinedot: '线条与点',