feat(web): desktop keyboard shortcuts

Space = play/pause, ←/→ = seek 5s, Shift+←/→ = prev/next, ↑/↓ = volume.
Handler lives in player.js (where _pos/_dur/seekToMs are in scope), wired
from startApp via initKeyboardShortcuts(). Ignored while typing in inputs,
and yields Space to focused buttons/links so it never double-fires.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.js

@@ -9,7 +9,7 @@ import {
   initProgressScrub, updateProgress, updatePreviewProgress, stopPreview,
   toggleRadioMode, starCurrent, downloadCurrent, renderQueue, playPreview,
   toggleStar, dlSongLocal, _applyRadioBtn, _applyShuffleRepeat, _restoreLastPlayed, setPlayIcon,
-  npToggleStar, updateMediaSession, updateNpColorMode, reconcileNativeState,
+  npToggleStar, updateMediaSession, updateNpColorMode, reconcileNativeState, initKeyboardShortcuts,
 } from './player/player.js';
 import {
   navigate, _navGoBack, _navLoad, onHeaderSearch, closeHeaderSearch,
@@ -220,6 +220,7 @@ window.addEventListener('load', () => {
 
   injectPWA();
   initProgressScrub();
+  initKeyboardShortcuts();
 
   // Click player-art to open Now Playing
   document.getElementById('player-art')?.addEventListener('click', openNowPlaying);