Phase 11: remote UX & network attach (0.11.0)

- Live remote-stack logs over a WebSocket proxied through the central app to
  the agent (/ws/agent-logs/{agent}/{stack}); agent gains a WS log endpoint.
- Deploy to a remote host from the UI: host selector in the New Stack editor
  and template dialog; templates instantiate onto an agent via the proxy.
- Network attach/detach: expandable inspect view per network with
  connect/disconnect + container picker; GET /{id}/containers, POST connect/disconnect.
- Remove dead pages/Placeholder.tsx.

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

backend/models/template.py

@@ -55,3 +55,4 @@ class TemplateSaveRequest(SQLModel):
 class TemplateInstantiateRequest(SQLModel):
     name: str  # new stack name
     values: dict[str, str] = {}
+    agent_id: int | None = None  # None = local host; otherwise deploy to a remote agent