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/static/frame_layout.js

@@ -280,12 +280,18 @@ window.addEventListener('resize', () => {
 // a frame can now have several widgets of the same type, so "the
 // Calendar tab" stopped meaning anything unambiguous.
 
-// widget_dialog_{photos,calendar,whiteboard}.js each define an
+// widget_dialog_{photos,calendar,whiteboard,tasks}.js each define an
 // init<Type>Dialog()/close<Type>Dialog() pair (loaded unconditionally by
 // frame_layout.html, since which one runs depends on which widget's gear
 // icon was clicked).
-const DIALOG_INIT = { photos: initPhotosDialog, calendar: initCalendarDialog, whiteboard: initWhiteboardDialog };
-const DIALOG_CLOSE = { photos: closePhotosDialog, calendar: closeCalendarDialog, whiteboard: closeWhiteboardDialog };
+const DIALOG_INIT = {
+  photos: initPhotosDialog, calendar: initCalendarDialog, whiteboard: initWhiteboardDialog,
+  tasks: initTasksDialog,
+};
+const DIALOG_CLOSE = {
+  photos: closePhotosDialog, calendar: closeCalendarDialog, whiteboard: closeWhiteboardDialog,
+  tasks: closeTasksDialog,
+};
 
 let openDialogWidgetType = null;