feat(settings): one page per section with a grouped nav and search

Settings moves from a tab bar to /settings/<section>: a left nav grouped
General / Agents / Routing / Git / Projects (a select on narrow screens), a
search box over every section (Ctrl/Cmd+F, Enter opens the first match), and
row primitives. Each AI tool has its page under agents/<id>, the AI Tools
modal becomes the Agents page, Pricing shows every vendor, Git → Worktrees
sets which untracked worktrees the sidebar lists, and each project gets a
page with its own worktree override and Forget all. ?tool= and #ai-tools
links redirect to the path form.

client/src/components/settings/AiToolsSettings.tsx

@@ -1,7 +1,6 @@
 /**
  * @file AiToolsSettings.tsx
- * @description The body of the AI Tools modal (Settings tab bar → AI Tools,
- * `AiToolsModal.tsx`): every AI coding CLI in the server's catalog
+ * @description Settings → Agents → AI tools (`/settings/agents`): every AI coding CLI in the server's catalog
  * (GET /api/ai-tools). Installed ones are rows with an enable switch, a
  * Default badge or Set default, and Update whenever the catalog names an
  * updater — no version lookup, the updater itself knows whether it has work.