fix: notification center always on top

The notification dropdown was at z-index: 100, well below fullscreen
terminals (1000), modal overlays (1000), and dialog overlays (1000).
Because .notification-center creates a stacking context with no explicit
z-index, the dropdown was trapped behind any of those overlays and
became unclickable.

- Set .notification-center z-index to 9999 so it competes above all
  other overlays in the root stacking context.
- Set .notification-dropdown z-index to 9999 for consistency.

Quality gates: tsc --noEmit (pass), build (pass)

apps/web/src/styles.css

@@ -4626,6 +4626,7 @@ a:active,
 .notification-center {
 	position: relative;
 	display: inline-flex;
+	z-index: 9999;
 }
 
 .notification-bell {
@@ -4679,7 +4680,7 @@ a:active,
 	border: 1px solid var(--border);
 	border-radius: 12px;
 	box-shadow: 0 8px 24px rgba(0, 0, 0, 0.12);
-	z-index: 100;
+	z-index: 9999;
 	display: flex;
 	flex-direction: column;
 	overflow: hidden;