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/templates/_widget_dialog_tasks.html

@@ -45,9 +45,13 @@
 </section>
 
 <section class="card" style="margin-top: 20px;">
-  <h2 class="card-title">Recently completed</h2>
+  <h2 class="card-title">Settings</h2>
   <form id="tasks-config-form">
-    <div class="checkbox-row">
+    <label>Name
+      <input type="text" id="tasks_name" maxlength="40" placeholder="Tasks" value="{{ task_cfg.name }}">
+    </label>
+    <p class="sub" style="margin-top: 4px;">Shown at the top of this widget on the panel instead of "Tasks" -- e.g. "Chores" or "Mom's list".</p>
+    <div class="checkbox-row" style="margin-top: 10px;">
       <input type="checkbox" id="tasks_show_completed" {% if task_cfg.show_completed %}checked{% endif %}>
       <label for="tasks_show_completed">Also show tasks completed in the last 24 hours</label>
     </div>