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/player/player.js

@@ -898,6 +898,56 @@ export function seekToMs(ms) {
   }
 }
 
+// ─── DESKTOP KEYBOARD SHORTCUTS ───
+// Space = play/pause · ←/→ = seek 5s · Shift+←/→ = prev/next · ↑/↓ = volume.
+// Skipped while typing in an input so it never hijacks the search/playlist fields.
+export function initKeyboardShortcuts() {
+  document.addEventListener('keydown', (e) => {
+    if (e.ctrlKey || e.altKey || e.metaKey) return;            // leave OS/browser combos alone
+    const t = e.target;
+    if (t && (t.tagName === 'INPUT' || t.tagName === 'TEXTAREA' ||
+              t.tagName === 'SELECT' || t.isContentEditable)) return;
+
+    switch (e.code) {
+      case 'Space':
+        // Don't hijack Space when a button/link is focused — it would double-fire.
+        if (t && (t.tagName === 'BUTTON' || t.tagName === 'A' ||
+                  t.closest?.('button, a, [role="button"]'))) return;
+        e.preventDefault();
+        togglePlay();
+        break;
+      case 'ArrowRight':
+        e.preventDefault();
+        if (e.shiftKey) nextSong(); else _seekBy(5);
+        break;
+      case 'ArrowLeft':
+        e.preventDefault();
+        if (e.shiftKey) prevSong(); else _seekBy(-5);
+        break;
+      case 'ArrowUp':
+        e.preventDefault();
+        _bumpVolume(5);
+        break;
+      case 'ArrowDown':
+        e.preventDefault();
+        _bumpVolume(-5);
+        break;
+    }
+  });
+}
+
+function _seekBy(deltaSec) {
+  const dur = _dur();
+  if (!dur) return;
+  seekToMs(Math.max(0, Math.min(dur, _pos() + deltaSec)) * 1000);
+}
+
+function _bumpVolume(deltaSlider) {
+  // setVolume takes a 0–100 slider value (squared internally); recover it from the actual volume.
+  const cur = Math.round(Math.sqrt(Math.max(0, S.audio.volume || 0)) * 100);
+  setVolume(Math.max(0, Math.min(100, cur + deltaSlider)));
+}
+
 function _initBarScrub(bar, fill, onMove, noTouch = false) {
   if(!bar || !fill) return;
   function getPos(clientX) { const r = bar.getBoundingClientRect(); return Math.max(0, Math.min(1, (clientX-r.left)/r.width)); }