fix(panels): guard removeEventListener against null _onResizeDebounced

Dropping the ?? this.ensureCorrectZones fallback in f9d6ed4 passed
_onResizeDebounced (which can be null) directly to removeEventListener.
That broke typecheck (TS2769) because removeEventListener does not
accept null.

Guard both calls with an if so the listener is only removed when we
actually have one registered. destroy() keeps the optional-chained
cancel() + null assignment to satisfy the source-level wiring test.

Verified: typecheck clean, 12/12 tests pass.

Author: fuleinist

src/app/panel-layout.ts

@@ -417,7 +417,9 @@ export class PanelLayoutManager implements AppModule {
     // Reset checkout overlay so next layout init can register its callback
     destroyCheckoutOverlay();
 
-    window.removeEventListener('resize', this._onResizeDebounced);
+    if (this._onResizeDebounced) {
+      window.removeEventListener('resize', this._onResizeDebounced);
+    }
     this._onResizeDebounced?.cancel();
     this._onResizeDebounced = null;
   }
@@ -1688,8 +1690,10 @@ export class PanelLayoutManager implements AppModule {
       });
     }
 
-    window.removeEventListener('resize', this._onResizeDebounced);
-    this._onResizeDebounced?.cancel();
+    if (this._onResizeDebounced) {
+      window.removeEventListener('resize', this._onResizeDebounced);
+      this._onResizeDebounced.cancel();
+    }
     this._onResizeDebounced = debounce(() => this.ensureCorrectZones(), 100);
     window.addEventListener('resize', this._onResizeDebounced);