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/pages/__tests__/screens.snapshot.test.tsx

@@ -585,6 +585,9 @@ describe("screen snapshots", () => {
   it("Settings", async () => {
     await snapshot(<Settings />, "/settings");
   });
+  it("Settings — notifications", async () => {
+    await snapshot(<Settings section="notifications" />, "/settings/notifications");
+  });
   it("Not found", async () => {
     await snapshot(<NotFound />, "/nope");
   });