feat(web): choose the default agent preset from General settings

One settings row picking which preset new sessions are composed from.

It is deliberately a new-session preference, not a live switch: a session's
preset is fixed at creation and the host refuses to adopt an existing session
under a different one, so the row says "applies to sessions you start from now
on" rather than implying it can retune a running agent.

Options and the current value come from one `agentPreset.list` call — the
roster already reports which id an unspecified session gets, so the row needs
no settings-schema introspection, unlike the permission row it is modelled on.
The write targets only the namespace's `default` field.

The menu marks `user` rows: a locally authored preset is exactly as privileged
as the plugins it names, and presenting it identically to a shipped one would
hide that.

An empty roster reads as `unavailable` and renders nothing, because composing
no presets is a valid deployment rather than a failure — distinct from a
roster call that failed, which surfaces its message.

pnpm-lock.yaml

@@ -146,6 +146,9 @@ importers:
       '@deepseek-ai/dsh-base':
         specifier: workspace:^
         version: link:../../packages/bundle/base
+      '@deepseek-ai/dsh-client-ui-agent-preset':
+        specifier: workspace:^
+        version: link:../../packages/client/ui-agent-preset
       '@deepseek-ai/dsh-headless':
         specifier: workspace:^
         version: link:../../packages/bundle/headless
@@ -1432,6 +1435,36 @@ importers:
         specifier: ^18.2.0
         version: 18.3.1(react@18.3.1)
 
+  packages/client/ui-agent-preset:
+    devDependencies:
+      '@deepseek-ai/dsh-client-connection':
+        specifier: workspace:^
+        version: link:../connection
+      '@deepseek-ai/dsh-client-locale':
+        specifier: workspace:^
+        version: link:../locale
+      '@deepseek-ai/dsh-client-runtime':
+        specifier: workspace:^
+        version: link:../runtime
+      '@deepseek-ai/dsh-client-ui-primitives':
+        specifier: workspace:^
+        version: link:../ui-primitives
+      '@deepseek-ai/dsh-client-ui-slots':
+        specifier: workspace:^
+        version: link:../ui-slots
+      '@deepseek-ai/dsh-invariants':
+        specifier: workspace:^
+        version: link:../../support/invariants
+      '@types/react':
+        specifier: ~18.3.1
+        version: 18.3.31
+      cordis:
+        specifier: ^4.0.0-rc.7
+        version: link:../../../vendor/cordis
+      react:
+        specifier: ^18.2.0
+        version: 18.3.1
+
   packages/client/ui-command:
     dependencies:
       clsx: