fix(tabs): put child follow behind a setting, stop unpinning children on order PUTs

Review fixes for "place new, child and web tabs into groups".

- Spawned tabs follow their parent only with the new synced setting
  spawnedTabsFollowParent (App Settings > Appearance > Tabs, off by default).
  TabLayoutService.sessionCreated() reads it fresh through a
  childrenFollowParent dep: off, a spawned session lands after its parent and
  is marked manual in the same write; on, it follows until moved by hand. The
  row menu shows the placement note and "Follow parent again" only when on.
  "New session" in a group menu is unaffected. zh-CN label and a
  Settings-Reference row.
- applyLegacySessionRank marks a child manual only when the requested order
  splits it from its parent's block. A PUT of the current order, or one that
  moves a family together, leaves every stored placement alone.
- The browser never echoes a create or delete through PUT /api/session-order,
  with or without groups: the layout service already re-projects the order and
  broadcasts it, and syncSessionOrder adopts it.
- The placement note in the row menu is plain text described by the menu, not
  a focusable menu item.
- A tab that first appears inside a collapsed group briefly marks that group's
  header.
- i18n skips .tab-name, so tab names are never translated.
- Tests: legacy-order cases for an unchanged order, a family reorder and a
  split; service cases for the setting and for order PUTs; the scheduled
  registration test now exercises register/undo; the grouped-create test binds
  port 0; one route case runs through the real layout service.

test/scheduled-layout-registration.test.ts

@@ -6,9 +6,17 @@ describe('scheduled session layout registration', () => {
   it('registers the layout before lifecycle work and rolls back a rejected tentative session', async () => {
     const sessions = new Map<string, { id: string; owner?: string }>();
     const session = { id: 'scheduled-session', owner: 'alice' };
-    const sessionCreated = vi.fn(async () => {
-      throw new Error('layout unavailable');
-    });
+    let registeredBeforeFailure = false;
+    // The layout service registers the session inside its owner lock, then the
+    // write fails and it calls the undo it was handed.
+    const sessionCreated = vi.fn(
+      async (_owner: string, _id: string, _placement: unknown, register: () => void | (() => void)) => {
+        const undo = register();
+        registeredBeforeFailure = sessions.get(session.id) === session;
+        if (typeof undo === 'function') undo();
+        throw new Error('layout unavailable');
+      }
+    );
     const server = Object.create(WebServer.prototype) as {
       sessions: typeof sessions;
       tabLayouts: { sessionCreated: typeof sessionCreated };
@@ -19,7 +27,8 @@ describe('scheduled session layout registration', () => {
 
     await expect(server.registerSessionWithLayout(session)).rejects.toThrow('layout unavailable');
 
-    expect(sessionCreated).toHaveBeenCalledWith('alice');
+    expect(sessionCreated).toHaveBeenCalledWith('alice', 'scheduled-session', undefined, expect.any(Function));
+    expect(registeredBeforeFailure).toBe(true);
     expect(sessions.has(session.id)).toBe(false);
   });