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/migrations/versions/c7e1a4f2d3b8_tree_home_person.py

@@ -0,0 +1,33 @@
+"""tree.home_person_id (per-tree default/home person)
+
+Revision ID: c7e1a4f2d3b8
+Revises: b3d5f8a1c920
+Create Date: 2026-06-07
+
+"""
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "c7e1a4f2d3b8"
+down_revision: str | None = "b3d5f8a1c920"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column("trees", sa.Column("home_person_id", sa.Uuid(), nullable=True))
+    op.create_foreign_key(
+        "fk_trees_home_person_id",
+        "trees",
+        "persons",
+        ["home_person_id"],
+        ["id"],
+        ondelete="SET NULL",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("fk_trees_home_person_id", "trees", type_="foreignkey")
+    op.drop_column("trees", "home_person_id")