a11y: Tasks 2-6 -- App shell, Titlebar, ToggleSwitch, Stepper, animate.ts

- Add skip-to-content link and dynamic document title (App.svelte)
- Wrap titlebar in header landmark, enlarge traffic lights to 44px (Titlebar.svelte)
- Enlarge toggle switch to 52x28, improve OFF knob contrast (ToggleSwitch.svelte)
- Enlarge stepper buttons to 36px, add keyboard hold-to-repeat (Stepper.svelte)
- Add keyboard feedback to pressable, focus glow to glowHover (animate.ts)

src/lib/components/Stepper.svelte

@@ -55,20 +55,34 @@
     if (holdTimer) { clearTimeout(holdTimer); holdTimer = null; }
     if (holdInterval) { clearTimeout(holdInterval as unknown as ReturnType<typeof setTimeout>); holdInterval = null; }
   }
+
+  function handleKeydown(fn: () => void, e: KeyboardEvent) {
+    if (["Enter", " "].includes(e.key)) {
+      e.preventDefault();
+      startHold(fn);
+    }
+  }
+
+  function handleKeyup(e: KeyboardEvent) {
+    if (["Enter", " "].includes(e.key)) {
+      stopHold();
+    }
+  }
 </script>
 
 <div class="flex items-center gap-1.5" role="group" aria-label={label}>
   <button
     type="button"
     aria-label="Decrease"
-    class="flex h-7 w-7 items-center justify-center rounded-lg
-           bg-[#141414] text-[#8a8a8a] transition-colors
+    class="flex h-9 w-9 min-h-[44px] min-w-[44px] items-center justify-center rounded-lg
+           border border-[#3a3a3a] bg-[#1a1a1a] text-text-sec transition-colors
            hover:bg-[#1c1c1c] hover:text-white
            disabled:opacity-20"
     onmousedown={() => startHold(decrement)}
     onmouseup={stopHold}
     onmouseleave={stopHold}
-    onclick={(e) => { if (e.detail === 0) decrement(); }}
+    onkeydown={(e) => handleKeydown(decrement, e)}
+    onkeyup={handleKeyup}
     disabled={value <= min}
   >
     &minus;
@@ -79,14 +93,15 @@
   <button
     type="button"
     aria-label="Increase"
-    class="flex h-7 w-7 items-center justify-center rounded-lg
-           bg-[#141414] text-[#8a8a8a] transition-colors
+    class="flex h-9 w-9 min-h-[44px] min-w-[44px] items-center justify-center rounded-lg
+           border border-[#3a3a3a] bg-[#1a1a1a] text-text-sec transition-colors
            hover:bg-[#1c1c1c] hover:text-white
            disabled:opacity-20"
     onmousedown={() => startHold(increment)}
     onmouseup={stopHold}
     onmouseleave={stopHold}
-    onclick={(e) => { if (e.detail === 0) increment(); }}
+    onkeydown={(e) => handleKeydown(increment, e)}
+    onkeyup={handleKeyup}
     disabled={value >= max}
   >
     +