fix(web): persist the default agent preset, and give its page an identity

Three fixes to the settings surface.

The default never reached settings.yaml. The api-proxy keeps an explicit
allowlist of settings namespaces it exposes to configuration clients, and
`agent-presets` was never added — so both pickers moved and then silently
forgot, which is worse than refusing the control. The host seam was fine all
along; only the wire boundary refused. The regression test fails with the
namespace removed.

The nav row showed the fallback gear and read `Agent preset` in Chinese,
matching neither sibling (通用设置 / 模型). It is now 智能体 with the think
glyph, the only unused icon in the set whose semantics point at the agent
rather than at tuning sliders (Personalization is already the workspace
browser's filter control).

packages/client/ui-agent-preset/src/client/locales.ts

@@ -59,7 +59,7 @@ export const zh: Record<AgentPresetSettingsKey, string> = {
   userTrust: '本地',
   seatHint: '本会话的 agent preset —— 发送第一条消息前可切换',
   lockedHint: '会话开始后，其 agent preset 即固定',
-  nav: 'Agent preset',
+  nav: '智能体',
   sectionIntro: 'preset 即一个会话的 agent 所运行的插件组装 —— 它的工具、提示词与能力。内置 preset 只读；复制一份即可改成自己的。',
   builtIn: '内置',
   defaultBadge: '默认',