fix(updates): scan all filesystem stacks for image updates (#514)

ImageUpdateService previously discovered stacks by iterating Docker
containers, which meant stacks without containers (e.g. after
docker compose down) were silently excluded from update checks.

Switch to a hybrid discovery approach: enumerate stacks from the
filesystem via FileSystemService.getStacks(), parse compose files
for image refs with .env variable resolution, then augment with
container-based image discovery for running stacks.

Also cleans up stale stack_update_status entries when stacks are
deleted or no longer exist on disk, and replaces the plain
update-available tooltip with an animated cursor follow pattern.

backend/src/index.ts

@@ -3615,6 +3615,9 @@ app.delete('/api/stacks/:name', async (req: Request, res: Response) => {
     // Stage 2: Obliterate the files
     await FileSystemService.getInstance(req.nodeId).deleteStack(stackName);
 
+    // Prevent stale update badges for deleted stacks
+    DatabaseService.getInstance().clearStackUpdateStatus(req.nodeId, stackName);
+
     invalidateNodeCaches(req.nodeId);
     res.json({ success: true });
   } catch (error: any) {