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/grid.py

@@ -22,11 +22,14 @@ GRID_SHORT = 5
 # the placement UI and server-side (routers/api_widgets.py). A calendar
 # widget crammed into 1x1 would be illegible regardless of size-tier
 # scaling (see calendar_render.py); whiteboard needs enough room to be
-# worth looking at; photos can go as small as a single cell.
+# worth looking at; photos can go as small as a single cell; tasks needs
+# enough width for a due-date prefix plus a couple words of summary
+# without truncating on every row.
 MIN_FOOTPRINT: dict[str, tuple[int, int]] = {
     "photos": (1, 1),
     "calendar": (3, 2),
     "whiteboard": (2, 2),
+    "tasks": (2, 2),
 }
 
 Rect = tuple[int, int, int, int]  # (x, y, w, h)