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>

backend/agent_app.py

@@ -62,7 +62,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.21.3"
+AGENT_VERSION = "0.21.4"
 
 
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