Account menu + Settings (change password); per-tree home person; full-width tree

- Sidebar bottom-left now shows the signed-in user; clicking opens a menu with
  Settings and Sign out. New /settings page: account info + change password
  (POST /auth/change-password, re-verifies current password). Export/restore/
  delete are stubbed there for the next pass.
- Per-tree default/home person: tree.home_person_id (migration) + TreeUpdate/
  Read; the tree and family views open focused on it; the person page gets a
  "Set as default" control and "Default person" badge. Cleared if that person
  is deleted. Complements the account-level "this is me" link.
- Tree visualization now fills the content area (AppShell drops the max-width
  column on the /tree route); other pages stay centered.
- Audit records are coerced JSON-safe (UUIDs/enums), so PATCHing UUID fields
  like home_person_id audits cleanly.

50 backend tests pass; migration up/down verified; frontend builds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/person_service.py

@@ -282,11 +282,14 @@ async def delete_person(
         await _soft_delete_one(session, actor=actor, tree=tree, person=p, now=now)
 
     # Soft delete leaves the row in place, so the DB-level "ON DELETE SET NULL"
-    # never fires — clear any account's self-person link to a deleted person.
+    # never fires — clear any links (account self-person, tree home person) to a
+    # deleted person.
+    deleted_ids = [p.id for p in to_delete]
     await session.execute(
-        update(User)
-        .where(User.self_person_id.in_([p.id for p in to_delete]))
-        .values(self_person_id=None)
+        update(User).where(User.self_person_id.in_(deleted_ids)).values(self_person_id=None)
+    )
+    await session.execute(
+        update(Tree).where(Tree.home_person_id.in_(deleted_ids)).values(home_person_id=None)
     )
 
     await session.commit()