Let a tasks widget have a custom on-panel name

Adds TaskWidgetConfig.name (migration 19, plain column add) shown at
the top of the widget on the actual panel instead of the hardcoded
"Tasks" header -- e.g. "Chores" or "Mom's list". The only widget type
with its own on-panel title at all, since it's the only one where
"which list is this" isn't already obvious from a calendar/photo/
whiteboard's own content.

Threaded through calendar_render's _draw_tasks/_build_tasks/
render_tasks/render_tasks_preview_png as a `title` param (default
"Tasks", truncated to fit -- a long custom name shouldn't be able to
overflow the widget's box), the config-save endpoint (tasks_name,
truncated server-side to a sane header length rather than rejected),
and a new "Settings" section in the tasks dialog.

Verified live in the browser: the name actually renders at the top of
the real composited panel (not just the dialog preview, which stays
gated on having a configured source), and persists correctly on both
desktop and mobile. Full suite (196 tests) passes.

server/tests/test_migrations.py

@@ -67,12 +67,14 @@ def test_expected_columns_exist_on_current_schema():
     inspector = inspect(db_module.engine)
     user_columns = {c["name"] for c in inspector.get_columns("users")}
     frame_columns = {c["name"] for c in inspector.get_columns("frames")}
+    task_widget_columns = {c["name"] for c in inspector.get_columns("task_widget_configs")}
 
     assert "webdav_base_url" in user_columns  # migration 15
     assert "webdav_username" in user_columns  # migration 14
     assert "calendar_caldav_url" in user_columns
     assert "whiteboard_cached_image" in frame_columns  # migration 14
     assert "calendar_week_start_offset" in frame_columns
+    assert "name" in task_widget_columns  # migration 19
 
 
 # --- widget system backfill (migration 16 + _ensure_widgets_backfilled) ---