feat(mini-player): optional preload toggle for Linux + macOS

Adds a "Preload mini player" toggle in Settings → General → App behaviour
(off by default). When enabled, the mini webview is built hidden at app
start so the first open is instant, matching the Windows experience.
Costs one extra WebKit process running in the background (~50–100 MB).

Windows already pre-creates the mini unconditionally as a hang workaround
(commit 71fbc717); the toggle is hidden there and the invoke is skipped,
so that platform is untouched.

- authStore: new `preloadMiniPlayer: boolean` (default false) + setter.
- lib.rs: new `preload_mini_player` command; idempotent, no-op when the
  window already exists. Registered in the invoke handler.
- App.tsx: main window invokes `preload_mini_player` when the toggle is
  on and the platform is not Windows.
- Settings.tsx: toggle row under "Minimize to Tray", gated with
  `!IS_WINDOWS` so Windows users don't see it.
- i18n: `preloadMiniPlayer` + `preloadMiniPlayerDesc` added to all 8
  locales (en, de, es, fr, nb, nl, ru, zh).

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

src/locales/zh.ts

@@ -538,6 +538,8 @@ export const zhTranslation = {
     showTrayIconDesc: '在系统通知区域 / 菜单栏显示 Psysonic 图标。',
     minimizeToTray: '最小化到托盘',
     minimizeToTrayDesc: '关闭窗口时，Psysonic 将继续在系统托盘中运行，而不是退出。',
+    preloadMiniPlayer: '预加载迷你播放器',
+    preloadMiniPlayerDesc: '在应用启动时于后台构建迷你播放器窗口，使其首次打开即可立即显示内容。会占用少量额外内存。',
     linuxWebkitSmoothScroll: '滚轮平滑（Linux）',
     linuxWebkitSmoothScrollDesc: '开：惯性滚动。关：逐行，类似 GTK。',
     discordRichPresence: 'Discord Rich Presence',