feat(web): pick a session's agent preset from the composer

A seat in the composer tool row, left of the model select, showing the preset
THIS session runs — its own recorded one, not the deployment default, because a
resumed session runs what it was created with. `SessionSummary` carries
`agentPreset` for that, alongside `cwd` and `origin`.

The switch exists only while the conversation has not started. After the first
turn the seat becomes a plain label rather than a disabled menu: a greyed
control reads as "temporarily unavailable", when in fact the choice is gone for
good. The seat never asks in that state, and the host refuses independently
with `agent-preset-locked`, so a stale client cannot slip one past it.

A rejected switch restores the previous value and surfaces the host's message
rather than leaving the seat showing something the session is not running.

pnpm-lock.yaml

@@ -1446,6 +1446,9 @@ importers:
       '@deepseek-ai/dsh-client-runtime':
         specifier: workspace:^
         version: link:../runtime
+      '@deepseek-ai/dsh-client-ui-conversation':
+        specifier: workspace:^
+        version: link:../ui-conversation
       '@deepseek-ai/dsh-client-ui-primitives':
         specifier: workspace:^
         version: link:../ui-primitives