perf(zoom): only register wheel listener while Ctrl/Meta is held

The zoom-on-Ctrl-wheel handler was registered on `window` for the
lifetime of the app with `{ passive: false, capture: true }`. That
flag combination forces the browser to wait for the handler to return
on EVERY wheel event before applying scroll — even though the handler
exits at line 1 when no modifier is held. The user reported a
characteristic input lag: scrollbar-drag was perfectly smooth, mouse-wheel
was choppy. His description ("como si la ventana principal lo cachara y
lo mandara al div interno y eso causara ese input lag") was literal —
the capture-phase non-passive listener intercepts the event at window
before routing to the scroll container.

Refactor: track Ctrl/Meta state via keydown/keyup and register the
non-passive wheel listener only while a modifier is actually held.
Default state = no wheel listener at all → browser native fast-path.
`window blur` releases the listener if the user alt-tabs with Ctrl held
(otherwise keyup never fires).

cc

src/lib/app/bootstrap.ts

@@ -75,10 +75,59 @@ function handleZoomWheel(event: WheelEvent): void {
   void applyZoomLevel(nextZoom);
 }
 
+/**
+ * Zoom-on-Ctrl-wheel handling.
+ *
+ * Previously this registered a non-passive, capture-phase `wheel` listener
+ * on `window` for the lifetime of the app. The non-passive flag (required
+ * so `preventDefault` actually stops the WebView's native Ctrl+wheel zoom)
+ * also tells the browser "wait for my handler before applying scroll" —
+ * which kills the native fast-path for *every* wheel event in the app,
+ * not just zoom-modified ones. The user perceived this as choppy mouse-wheel
+ * scrolling vs perfectly-smooth scrollbar-drag scrolling.
+ *
+ * New approach: the wheel listener only exists while Ctrl/Meta is held.
+ * Keydown registers it, keyup (or window blur) unregisters it. Net:
+ *   - 99% of normal scrolling: no wheel listener at all, browser fast-path
+ *   - During zoom (Ctrl held): non-passive listener active, preventDefault
+ *     still blocks native zoom and our `applyZoomLevel` runs
+ */
+const ZOOM_WHEEL_OPTIONS: AddEventListenerOptions = { passive: false, capture: true };
+let zoomWheelActive = false;
+
+function registerZoomWheel(): void {
+  if (zoomWheelActive) return;
+  zoomWheelActive = true;
+  window.addEventListener('wheel', handleZoomWheel, ZOOM_WHEEL_OPTIONS);
+}
+
+function unregisterZoomWheel(): void {
+  if (!zoomWheelActive) return;
+  zoomWheelActive = false;
+  window.removeEventListener('wheel', handleZoomWheel, ZOOM_WHEEL_OPTIONS);
+}
+
+function handleZoomKeyDown(e: KeyboardEvent): void {
+  if (e.ctrlKey || e.metaKey) registerZoomWheel();
+}
+
+function handleZoomKeyUp(e: KeyboardEvent): void {
+  // After any keyup, if neither modifier is held anymore, drop the listener.
+  if (!e.ctrlKey && !e.metaKey) unregisterZoomWheel();
+}
+
 export function setupZoomControls(): () => void {
-  const options: AddEventListenerOptions = { passive: false, capture: true };
-  window.addEventListener('wheel', handleZoomWheel, options);
-  return () => window.removeEventListener('wheel', handleZoomWheel, options);
+  window.addEventListener('keydown', handleZoomKeyDown);
+  window.addEventListener('keyup', handleZoomKeyUp);
+  // Releases the listener if the user alt-tabs with Ctrl held — otherwise
+  // the keyup never fires and we stay in the slow path.
+  window.addEventListener('blur', unregisterZoomWheel);
+  return () => {
+    window.removeEventListener('keydown', handleZoomKeyDown);
+    window.removeEventListener('keyup', handleZoomKeyUp);
+    window.removeEventListener('blur', unregisterZoomWheel);
+    unregisterZoomWheel();
+  };
 }
 
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