Let a tasks widget merge multiple task lists, checkbox+color like calendar

Tasks widgets could only ever point at one CalDAV task list (a radio-
button picker, owner-only). Now they merge any number of included task
lists across every linked user, same checkbox-inclusion + optional
pinned-color shape a calendar widget already has for its calendars --
FrameTaskList mirrors FrameCalendar exactly, down to the same owner-
adds/anyone-mutes permission split (api_widget_task_list_select/
api_widget_task_list_color). Reused calendar_render._event_colors/
_draw_color_bar as-is for the per-task color bar -- a task dict's
owner_display_name/color_index is exactly that function's single-
source fallback shape.

Also added an opt-in "show tasks completed in the last 24 hours"
toggle (TaskWidgetConfig.show_completed): caldav_client.fetch_tasks
now accepts a completed_since cutoff and returns completed VTODOs
(with their completion time) instead of silently dropping them, and
_draw_tasks gives a completed task a filled checkbox + muted text
instead of the normal empty-box/due-date row.

Migration 18 splits the single-source TaskWidgetConfig columns
(added by 17, splitting tasks out of the calendar widget in the first
place) into frame_task_lists, carrying forward each widget's existing
single source as its first included list -- same shape migration 9
used carrying forward frame_calendars' old single opt-in.

Verified live in the browser (desktop + mobile): the new "Included
task lists" + "Recently completed" dialog sections, the show_completed
toggle actually persisting through a real HTTP round-trip, and no
regression in the calendar widget's own "Included calendars" dialog.
Full suite (192 tests, including new merge_tasks/config_save/migration
coverage) passes.

server/app/templates/_widget_dialog_tasks.html

@@ -1,39 +1,58 @@
 <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>
+  <h2 class="card-title">Included task lists</h2>
+  <p class="sub">Each linked person adds their own CalDAV task lists (set
+    up in <a href="/settings">Settings</a> -- a plain ICS subscription
+    has no task list) -- being linked here doesn't include anything
+    automatically. Anyone linked to this frame can mute a list they'd
+    rather not see here, even one they don't own; only its owner can add
+    it back.</p>
+  <ul class="calendar-user-list">
+    {% for u in task_users %}
+      <li>
+        <p class="calendar-user-name">{{ u.display_name }}{% if u.is_self %} (you){% endif %}</p>
+        {% if u.task_lists %}
+          {% set current_palette = frame.palette_rgb or default_palette_rgb %}
+          {% set current_hex = palette_to_hex(current_palette) %}
+          {% for c in u.task_lists %}
+            <div class="checkbox-row calendar-row" style="margin-top: 6px;">
+              <input type="checkbox" class="task-list-toggle"
+                     data-user-id="{{ u.user_id }}" data-key="{{ c.key }}" data-label="{{ c.label }}"
+                     {% if c.included %}checked{% endif %}>
+              <label style="margin: 0; font-weight: normal;">{{ c.label }}</label>
+              {% if u.is_self %}
+                <span class="task-list-color-picker" data-key="{{ c.key }}">
+                  {% for idx in range(2, 6) %}
+                    <button type="button" class="color-swatch {% if c.color_index == idx %}selected{% endif %}"
+                            data-index="{{ idx }}" title="{{ task_color_labels[idx] }}"
+                            style="background-color: {{ current_hex[idx] }};"></button>
+                  {% endfor %}
+                  <button type="button" class="color-swatch color-swatch-auto {% if c.color_index is none %}selected{% endif %}"
+                          data-index="" title="Auto (assigned automatically)">Auto</button>
+                </span>
+              {% endif %}
+            </div>
+          {% endfor %}
+        {% elif u.is_self %}
+          <p class="sub" style="margin-top: 6px;">No CalDAV task lists set up yet -- add a CalDAV account in <a href="/settings">Settings</a>.</p>
+        {% else %}
+          <p class="sub" style="margin-top: 6px;">No task lists included.</p>
+        {% endif %}
+      </li>
+    {% endfor %}
+  </ul>
+</section>
 
-  <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 class="card" style="margin-top: 20px;">
+  <h2 class="card-title">Recently completed</h2>
+  <form id="tasks-config-form">
+    <div class="checkbox-row">
+      <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>
+    <button type="submit">Save</button>
+  </form>
 </section>
 
 <section class="card" style="margin-top: 20px;">