feat(web): delete a session from the sidebar

A session could be created from the sidebar but never removed, so the
chat surface accumulated every experiment anyone ever named. Each row
now carries a small delete affordance that only names the key: the
confirmation is the shell's, and it says the turn history goes with
the session before calling the new route. On confirm the chat moves to
the first remaining session — the stream's watcher follows the key —
so nothing keeps streaming a session that no longer exists.

The row had to stop being a single button, since a nested button is
invalid HTML: the select button and the delete button now sit in a
container that owns the hover and active surface.

web/claw/README.md

@@ -122,6 +122,14 @@ Two consequences worth knowing before changing this code:
   lets 400/409 land in the error banner. The phone client's status
   keys (`phone:…`) stay out of the inventory; the full list stays on
   the wire.
+- **Deleting a session is a decision, not a click.** The row's delete
+  button only names the key; the shell's dialog spells out that the
+  turn history goes with it, and `DELETE /v1/sessions/{key}` runs on
+  confirm — so a key created again under the same name starts empty.
+  A 409 (a turn holds the key) and a 404 (already gone) land in the
+  error banner like every other refusal, and the chat moves to the
+  first remaining session, the stream's watcher following the key so
+  nothing keeps streaming a session that no longer exists.
 
 ## Layout