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/models/tree.py

@@ -26,6 +26,16 @@ class Tree(Base, UUIDPrimaryKey, Timestamps, SoftDelete):
         default=TreeVisibility.private,
         server_default=TreeVisibility.private.value,
     )
+    # The person a tree opens focused on (its "home"/root person). Cleared if
+    # that person is deleted. use_alter + name: trees<->persons form an FK cycle.
+    home_person_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey(
+            "persons.id",
+            ondelete="SET NULL",
+            name="fk_trees_home_person_id",
+            use_alter=True,
+        )
+    )
 
 
 class TreeMembership(Base, UUIDPrimaryKey, Timestamps):