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>

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 = "0.21.1"
+AGENT_VERSION = "0.21.3"
 
 
 # --------------------------------------------------------------------------- #

backend/main.py

@@ -55,7 +55,7 @@ async def lifespan(app: FastAPI):
     schedule_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.21.2", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.21.3", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,