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/en.ts

@@ -555,6 +555,8 @@ export const enTranslation = {
     showTrayIconDesc: 'Display the Psysonic icon in the system notification area / menu bar.',
     minimizeToTray: 'Minimize to Tray',
     minimizeToTrayDesc: 'When closing the window, keep Psysonic running in the system tray instead of quitting.',
+    preloadMiniPlayer: 'Preload mini player',
+    preloadMiniPlayerDesc: 'Build the mini player window in the background at app start so it shows content instantly on first open. Uses a little extra memory.',
     discordRichPresence: 'Discord Rich Presence',
     discordRichPresenceDesc: 'Show the currently playing track on your Discord profile. Requires Discord to be running.',
     useCustomTitlebar: 'Custom title bar',