feat(changelog): replace auto-modal with sidebar banner + /whats-new page

Removes the giant startup changelog modal. After an update, a compact
neutral-palette pill now sits above Now Playing in the sidebar saying
"Changelog — vX.Y.Z". Clicking opens a proper /whats-new page in the
main content area; X dismisses. Page renders the current version's
CHANGELOG entry with inline Markdown (headings, lists, blockquotes,
hr, bold/italic/code, and real [label](url) links that open via the
Tauri shell plugin).

Visual tokens are hardcoded slate/gray/blue so the banner and page look
identical across every theme (dark, light, skeuomorphic, whatever) —
requested for consistent contrast. Auto-mark-as-seen removed from the
page; the banner only goes away on explicit dismiss, so users can
re-read as often as they want.

Settings → About gains a "Release notes" row with a link that resets
the seen-version and opens the page, so the banner can be retriggered
manually (also helpful for dev builds ahead of the current tag).

The existing "Show changelog on update" toggle now gates the banner
instead of the modal; description strings updated in all 8 locales.

fix(themes): WCAG contrast audit — mocha + winmedplayer + wista

Mocha (Catppuccin dark)
  .track-size used --ctp-overlay0 directly (3.36:1 on bg-app, 2.57
  on bg-card). Swapped to --text-muted → 7.37 / 5.65. Fix also lifts
  macchiato/frappe/latte which shared the failure.

WinMedPlayer (Luna)
  --text-muted #b8d0f8 (3.87:1 on bg-app) → #e8f0ff (5.28)
  --border   #2a5090 (1.31 on bg-app)   → #071027 (3.12, meets 3:1 UI)

Wista (Vista Aero)
  Lyrics pane sits on bg-sidebar #0e1e3e but used --text-primary
  (#0d1d3c) for active lines — 1.01:1, literally invisible. Added
  component overrides: .lyrics-line / .lyrics-status / word-synced
  variants → #aac8f0 (9.60), .lyrics-line.active → #ffffff (16.48).
  Palette: --warning #c8980c → #735a00 (2.37 → 5.91 on bg-app),
  --text-muted #4870a8 → #3f6aa0 (4.23 → 4.65 on bg-card).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/zh.ts

@@ -582,11 +582,13 @@ export const zhTranslation = {
     aboutVersion: '版本',
     aboutBuiltWith: '使用 Tauri · React · TypeScript · Rust/rodio 构建',
     aboutAiCredit: '在 Anthropic 的 Claude Code 支持下开发',
+    aboutReleaseNotesLabel: '版本说明',
+    aboutReleaseNotesLink: '打开此版本的新功能',
     aboutContributorsLabel: '贡献者',
     aboutSpecialThanksLabel: '特别感谢',
     changelog: '更新日志',
     showChangelogOnUpdate: '更新时显示"新功能"',
-    showChangelogOnUpdateDesc: '新版本首次启动时自动显示更新内容。',
+    showChangelogOnUpdateDesc: '更新后在「正在播放」上方显示一个低调的更新日志横幅。点击打开发行说明，X 按钮关闭。',
     randomMixTitle: '随机混音',
     randomMixBlacklistTitle: '自定义过滤关键词',
     randomMixBlacklistDesc: '当任何关键词匹配流派、标题、专辑或艺术家时，歌曲将被排除（当上方复选框开启时生效）。',
@@ -722,6 +724,14 @@ export const zhTranslation = {
     dontShowAgain: '不再显示',
     close: '知道了',
   },
+  whatsNew: {
+    title: '新功能',
+    empty: '此版本暂无更新日志。',
+    bannerTitle: '更新日志',
+    bannerCollapsed: 'v{{version}} 新功能',
+    dismiss: '关闭',
+    close: '关闭',
+  },
   help: {
     title: '帮助',
     s1: '入门指南',