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/stores.svelte.js

@@ -403,6 +403,19 @@ class AppState {
 		await this.update(id, { due_date: date, due_time: clock });
 	}
 
+	/**
+	 * Sets how long a task is, in minutes. Nothing else about it moves — the date
+	 * and time are left alone, so making a task longer on the calendar does not
+	 * also reschedule it.
+	 */
+	async resize(id, minutes) {
+		const previous = this.get(id);
+		if (!previous) return;
+		const rounded = Math.max(15, Math.round(minutes / 15) * 15);
+		if (previous.duration_min === rounded) return;
+		await this.update(id, { duration_min: rounded });
+	}
+
 	async remove(id) {
 		const previous = [...this.tasks];
 		this.tasks = this.tasks.filter((t) => t.id !== id);