feat(calendar): a task's length is drawn, resizable, and editable

A timed task was drawn at a fixed half hour whatever its `duration_min` said, so a
length the data model already carried was invisible and uneditable. Three parts:

- The calendar draws a task at its real length — a quarter-hour floor so a
  zero-minute task is still grabbable, the day's end as the ceiling — instead of a
  flat thirty minutes. A ninety-minute task now looks like ninety minutes.

- A grip along the foot of a timed task drags its length. Pointer events, not the
  HTML5 drag the body uses, and stopPropagation between them — otherwise making a
  task longer would pick it up and move it instead. It snaps to the quarter hour,
  follows the handle live, and writes once on release through a new store `resize`.
  Moving a task (the existing day/time drag) leaves the length alone.

- The task drawer gains a length field beside the time — minutes, shown only with
  a time to hang it on, since a duration without a start is a length of nothing and
  the calendar has nowhere to draw it. So a length can be set off the calendar too.

e2e: dragging the foot of a task makes it longer both on screen and in what the
server stores, and the time does not move. Full Chromium suite green.

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

web/src/lib/components/TaskDetail.svelte

@@ -23,6 +23,9 @@
 	let priority = $state(4);
 	let dueDate = $state('');
 	let dueTime = $state('');
+	// Length in minutes, as a string for the field. Only meaningful with a time —
+	// see the field below — so it is seeded and shown alongside the clock.
+	let duration = $state('');
 	let labels = $state('');
 	let projectId = $state('');
 	let recurrence = $state('');
@@ -57,6 +60,7 @@
 		priority = task.priority;
 		dueDate = task.due_date ?? '';
 		dueTime = task.due_datetime ? clockOf(task.due_datetime) : '';
+		duration = task.duration_min ? String(task.duration_min) : '';
 		labels = (task.labels ?? []).join(', ');
 		projectId = task.project_id;
 		recurrence = task.recurrence_rule ?? '';
@@ -436,6 +440,21 @@
 						disabled={!dueDate}
 						onchange={() => save({ due_date: dueDate, due_time: dueTime })}
 					/>
+					<!-- Length in minutes, and only with a time to hang it on: a duration
+					     without a start is a length of nothing, and the calendar has
+					     nowhere to draw it. Empty clears it. -->
+					<input
+						type="number"
+						class="dur"
+						min="0"
+						step="15"
+						aria-label={t('detail.duration')}
+						placeholder={t('detail.durationShort')}
+						bind:value={duration}
+						disabled={!dueTime}
+						onchange={() =>
+							save({ duration_min: duration === '' ? 0 : Math.max(0, Math.round(Number(duration))) })}
+					/>
 				</div>
 			</div>
 		</div>
@@ -731,6 +750,11 @@
 		width: 7.5rem;
 	}
 
+	.due input.dur {
+		flex: none;
+		width: 5rem;
+	}
+
 	.due input:disabled {
 		opacity: 0.5;
 	}