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/de.ts

@@ -553,6 +553,8 @@ export const deTranslation = {
     showTrayIconDesc: 'Psysonic-Icon im System-Tray / in der Menüleiste anzeigen.',
     minimizeToTray: 'Im Tray minimieren',
     minimizeToTrayDesc: 'Beim Schließen des Fensters läuft Psysonic weiter im System-Tray statt zu beenden.',
+    preloadMiniPlayer: 'Mini-Player vorladen',
+    preloadMiniPlayerDesc: 'Baut das Mini-Player-Fenster beim App-Start im Hintergrund auf, damit es beim ersten Öffnen sofort Inhalt zeigt. Kostet etwas mehr RAM.',
     discordRichPresence: 'Discord Rich Presence',
     discordRichPresenceDesc: 'Zeigt den aktuell gespielten Titel im Discord-Profil an. Discord muss dafür geöffnet sein.',
     useCustomTitlebar: 'Eigene Titelleiste',