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/app/widgets/tasks.py

@@ -23,7 +23,7 @@ from PIL import Image
 from sqlalchemy.orm import Session
 
 from ..calendar_render import _build_tasks
-from ..models import Frame, Widget
+from ..models import Frame, TaskWidgetConfig, Widget
 from ..routers.common import get_or_refresh_tasks_for_widget
 
 ACTION_LABELS: dict[str, str] = {}
@@ -35,7 +35,8 @@ def render(db: Session, frame: Frame, widget: Widget, target_w: int, target_h: i
     identical note; every widget type's render() shares one call
     signature regardless of which ones actually care."""
     tasks = get_or_refresh_tasks_for_widget(db, frame, widget)
-    return _build_tasks(tasks, target_w, target_h, frame.palette_rgb)
+    cfg = db.get(TaskWidgetConfig, widget.id)
+    return _build_tasks(tasks, target_w, target_h, frame.palette_rgb, cfg.name or "Tasks")
 
 
 ACTIONS: dict = {}