Split the tasks feature out of the calendar widget into its own widget type

Task lists used to be a week-view-only sub-feature bolted onto calendar
widgets (CalendarWidgetConfig.tasks_*), so a task list could only exist
tied to a calendar's view and only inside its footprint. Tasks are now
a standalone widget type (TaskWidgetConfig, app/widgets/tasks.py) that
can be placed and sized independently, same as photos/calendar/
whiteboard -- no separate "enabled" flag either, since being on the
grid at all is the on/off switch, matching every other widget type.

Migration 17 creates task_widget_configs, extracts any existing
calendar widget's configured task source into a new sibling tasks
widget (auto-placed in open grid space, source dropped+logged if truly
none is left), then drops calendar_widget_configs' now-dead tasks_*
columns in the same migration -- this project's usual same-migration-
drop convention. Also handles the rarer case of a database jumping
straight from before the widget system existed to after this split in
one boot, via the legacy Frame.calendar_tasks_* columns.

Verified live in the browser at desktop and mobile widths: adding a
Tasks widget, its own dialog (task-list source picker + preview), and
confirming the calendar widget's dialog no longer mentions tasks at
all. Full test suite (180 tests, including new coverage for the widget
render/actions, the migration's data-extraction path, and the
permission-boundary shape for tasks-source) passes.

server/app/templates/_widget_dialog_tasks.html

@@ -0,0 +1,44 @@
+<h2 class="dialog-title">Tasks widget</h2>
+
+<section class="card">
+  <h2 class="card-title">Task list source</h2>
+  <p class="sub">A simple outstanding-task checklist. CalDAV only -- a
+    task list is a VTODO collection, which a plain ICS subscription
+    doesn't carry.</p>
+
+  <div id="tasks-current-source">
+    {% if tasks_source %}
+      <p class="sub" style="margin-top: 10px;">
+        Currently using <strong>{{ tasks_source.display_name }}</strong>'s
+        <strong>{{ tasks_source.label }}</strong> list.
+        <button type="button" class="btn-inline secondary" id="tasks-source-clear">Clear</button>
+      </p>
+    {% else %}
+      <p class="sub" style="margin-top: 10px;">No task list configured yet.</p>
+    {% endif %}
+  </div>
+
+  {% if viewer_task_calendars %}
+    <p class="sub" style="margin-top: 10px;">Use one of your own CalDAV task lists:</p>
+    <ul class="calendar-user-list">
+      {% for c in viewer_task_calendars %}
+        <li class="checkbox-row" style="margin-top: 6px;">
+          <input type="radio" name="tasks-source-choice" class="tasks-source-choice" data-key="{{ c.key }}"
+                 {% if tasks_source and tasks_source.user_id == user.id and tasks_source.calendar_key == c.key %}checked{% endif %}>
+          <label style="margin: 0; font-weight: normal;">{{ c.label }}</label>
+        </li>
+      {% endfor %}
+    </ul>
+  {% else %}
+    <p class="sub" style="margin-top: 10px;">You don't have any CalDAV
+      task lists available -- set up a CalDAV account in
+      <a href="/settings">Settings</a> first.</p>
+  {% endif %}
+</section>
+
+<section class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Preview</h2>
+  <p class="sub">How this widget currently renders.</p>
+  <img class="preview-img" id="tasks-preview" alt="Tasks preview">
+  <button type="button" class="secondary" id="tasks-preview-refresh">Refresh now</button>
+</section>