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/app/components/settings/AboutPage.vue

@@ -13,6 +13,7 @@ const routes = [
   { method: 'GET', path: '/healthz', note: 'liveness, open without a token' },
   { method: 'GET', path: '/v1/sessions', note: 'the dispatch-key inventory' },
   { method: 'POST', path: '/v1/sessions', note: 'create an interactive session from a name' },
+  { method: 'DELETE', path: '/v1/sessions/{key}', note: 'delete a session and its turn history' },
   { method: 'POST', path: '/v1/sessions/{key}/messages', note: 'message ingest, 202 + stream' },
   { method: 'GET', path: '/v1/sessions/{key}/events', note: 'the turn-event stream (SSE)' },
   { method: 'POST', path: '/v1/consents/{id}', note: 'answer a consent (a secret or a grant)' },