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/widgets/tasks.py

@@ -1,12 +1,21 @@
-"""Tasks widget: a simple outstanding-task checklist in its own region --
-split out of the calendar widget's old week-view-only task list (see
-models.TaskWidgetConfig) so a task list can be placed and sized on its
-own, independent of any calendar's view/footprint.
+"""Tasks widget: a simple outstanding-task checklist merged from one or
+more of its linked users' CalDAV task lists, in its own region -- split
+out of the calendar widget's old week-view-only, single-list task list
+(see models.TaskWidgetConfig) so a task list can be placed and sized on
+its own, independent of any calendar's view/footprint, and can merge
+more than one person's list the same way a calendar widget merges more
+than one person's calendar (see models.FrameTaskList).
 
 No "enabled" concept and no button actions: the widget's mere presence
 on the grid is the on/off switch (same as every other widget type), and
 its cache refreshes on the same throttled schedule as weather -- nothing
-here to advance/back/force."""
+here to advance/back/force. No placeholder for "nothing included yet"
+either -- same posture as app/widgets/calendar.py, which this otherwise
+mirrors closely: render() always draws through get_or_refresh_tasks_
+for_widget's result even when it's [], showing "Nothing outstanding"
+rather than a distinct not-configured state (the dialog's preview
+endpoint is what actually 400s for that case, same asymmetry calendar
+already has)."""
 
 from __future__ import annotations
 
@@ -14,9 +23,8 @@ from PIL import Image
 from sqlalchemy.orm import Session
 
 from ..calendar_render import _build_tasks
-from ..models import Frame, TaskWidgetConfig, Widget
+from ..models import Frame, Widget
 from ..routers.common import get_or_refresh_tasks_for_widget
-from ._shared import placeholder_image
 
 ACTION_LABELS: dict[str, str] = {}
 
@@ -26,11 +34,8 @@ def render(db: Session, frame: Frame, widget: Widget, target_w: int, target_h: i
     """is_normal_wake is unused here -- see app/widgets/photos.py's
     identical note; every widget type's render() shares one call
     signature regardless of which ones actually care."""
-    cfg = db.get(TaskWidgetConfig, widget.id)
-    if not cfg.calendar_key or not cfg.user_id:
-        return placeholder_image(target_w, target_h, ["Tasks widget", "not configured yet"])
     tasks = get_or_refresh_tasks_for_widget(db, frame, widget)
-    return _build_tasks(tasks, target_w, target_h)
+    return _build_tasks(tasks, target_w, target_h, frame.palette_rgb)
 
 
 ACTIONS: dict = {}