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/tree_service.py

@@ -70,7 +70,7 @@ async def update_tree(
         raise NotFound("tree not found")
     if not await privacy.can_edit_tree(session, user_id=actor.id, tree=tree):
         raise Forbidden("not an editor of this tree")
-    for key in {"name", "description", "visibility"} & changes.keys():
+    for key in {"name", "description", "visibility", "home_person_id"} & changes.keys():
         setattr(tree, key, changes[key])
     record_audit(
         session,