feat: filter notifications to warnings/errors/ready only and add clear-all button

Notification filtering:
- lifecycle_hooks.py: only instance.error and instance.health_changed
  with status=running generate notifications. All other lifecycle events
  (created, started, stopped, restarted, deleted) are filtered out.
- health_monitor.py: only error and unhealthy states generate notifications.
  Running/recovered state no longer creates info notifications.
- _derive_title now maps instance.health_changed to "Container ready".

Clear-all button:
- Added dismiss_all() to NotificationService
- Added DELETE /notifications endpoint for bulk dismiss
- Frontend: clearAllNotifications API, clearAll in notification context,
  "Clear all" button in notification drawer alongside "Mark all as read"
- Added CSS for .notification-clear-all with danger hover state
- Updated notification-center tests

Quality gates: pytest (21 passed), vitest (11 passed)

apps/api/src/services/notification_service.py

@@ -195,6 +195,32 @@ class NotificationService:
         await session.commit()
         return result.rowcount or 0
 
+    async def dismiss_all(
+        self,
+        session: AsyncSession,
+        user_id: uuid.UUID,
+    ) -> int:
+        """Soft-delete all non-dismissed notifications for a user.
+
+        Args:
+            session: Database session.
+            user_id: Owner of the notifications.
+
+        Returns:
+            Number of rows updated.
+        """
+        stmt = (
+            update(Notification)
+            .where(
+                Notification.user_id == user_id,
+                Notification.dismissed_at.is_(None),
+            )
+            .values(dismissed_at=datetime.now(timezone.utc))
+        )
+        result: CursorResult[Any] = await session.execute(stmt)  # type: ignore[assignment]
+        await session.commit()
+        return result.rowcount or 0
+
     async def dismiss(
         self,
         session: AsyncSession,