Stacks page: table layout with usage meters, matching the dashboard (0.21.4)

The Stacks tab now lists stacks in the shared StacksTable (status, CPU
and memory meters, inline start/stop/restart) instead of cards, for both
the local host and per-agent sections — same look as the dashboard.
StacksTable gained optional showEdit/showDelete props so the management
surface keeps the Edit link and local Delete (with confirm). Search and
sort are unchanged. Removed the now-unused StackCard component.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.21.3",
+  "version": "0.21.4",
   "type": "module",
   "scripts": {
     "dev": "vite",