Picker review fixes: validate stream_id on stop; accurate offline + no-system copy

- POST /discord/nodes/{id}/streams/stop: stream_id must be guild:channel
  snowflakes, so a malformed id is a 422 instead of a Firestore path error.
- Offline footer said channels resume on reconnect; ad-hoc ones stop once
  the node is gone past the lease grace (reconcile_once). Say so.
- Picker: say why Start is disabled when the node has no system.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/app/routers/discord_web.py

@@ -274,7 +274,9 @@ async def start_node_stream(node_id: str, body: NodeStreamStart, decoded: dict =
 
 
 class NodeStreamStop(BaseModel):
-    stream_id: Optional[str] = None
+    # guild:channel snowflakes (leases.stream_id_for). Validated so a malformed
+    # id (e.g. one with "/") is a 422, not a Firestore path error / 500.
+    stream_id: Optional[str] = Field(None, pattern=r"^[0-9]{1,25}:[0-9]{1,25}$")
 
 
 @router.post("/nodes/{node_id}/streams/stop")