Refactor: extract shared Dashboard StacksTable component (0.21.3)

The Dashboard's stacks-usage table (CPU/mem meters + inline
start/stop/restart) is now a reusable components/stacks/StacksTable.tsx
used by both the local and per-agent host sections. Removes the
duplicate inline definition that was left behind by the half-finished
extraction (which broke the build: redeclared StacksTable + dangling
imports). No behaviour change.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.21.1",
+  "version": "0.21.3",
   "type": "module",
   "scripts": {
     "dev": "vite",