feat(tasks): undo a completion, and a place to find what was closed

Reported from use: "I just closed a task by accident, and there is no undo and no
list of closed tasks." Both halves were true.

Completing is one click on a small circle beside a row you were only reading, and
it takes the row off the screen — so the mistake and the evidence of it leave
together. There was no name left to search for and nowhere to look: the project
page can show its own closed tasks, but only if you already know which project it
was in.

The toast covers the mistake you notice at once. It names the task, so you can see
which one you hit, and stands for twelve seconds rather than five — five is enough
to read "could not save" and not enough to notice, decide and reach the mouse. A
toast with an action also gets two targets: the message dismisses and the action
acts, which were one button before and would otherwise put "yes, put it back" and
"go away" in the same place.

`/faerdige` covers the one you notice tomorrow. A place rather than a toggle in
every view, because somebody hunting for a task they closed does not know which
view they closed it in. Newest first — a record of what is finished is read
backwards from now, not by a due date the task no longer has — and grouped by the
day, because "I dag" and "I går" is how anybody would describe it.

A recurring task is offered no undo. Completing it moves it to its next date rather
than closing it, so there is nothing to reverse, and an undo that quietly rolled
the date back would be a different act from the one it claims to be.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/tasks.go

@@ -189,7 +189,7 @@ func (db *DB) ListTasks(ctx context.Context, userID string, f TaskFilter) ([]Tas
 		FROM tasks t
 		JOIN projects p ON p.id = t.project_id
 		WHERE ` + strings.Join(where, " AND ") + `
-		ORDER BY t.due_date IS NULL, t.due_date, t.priority, t.sort_order, t.created_at`
+		ORDER BY ` + order(f)
 
 	if f.Limit > 0 {
 		query += ` LIMIT ?`
@@ -346,6 +346,19 @@ const taskCounts = `
 	  WHERE c.parent_id = t.id AND c.deleted_at IS NULL AND c.completed_at IS NOT NULL),
 	(SELECT count(*) FROM attachments a WHERE a.task_id = t.id)`
 
+// order is how a list of tasks is sorted, which depends on what the list is for.
+//
+// A plan is read by when it is due and how urgent it is. A record of what has been
+// finished is read backwards from now — "what did I just do" and "what did I close
+// by mistake" are both questions about the most recent thing, and neither is
+// answered by a due date the task no longer has.
+func order(f TaskFilter) string {
+	if f.CompletedOnly {
+		return "t.completed_at DESC, t.id DESC"
+	}
+	return "t.due_date IS NULL, t.due_date, t.priority, t.sort_order, t.created_at"
+}
+
 func scanTask(rows *sql.Rows) (Task, error) {
 	var t Task
 	var sectionID, parentID, dueDate, dueTZ, recurrence, assignee, completedBy sql.NullString