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/locales/en.js

@@ -593,6 +593,8 @@ export const en = {
 	'detail.none': 'None',
 	'detail.due': 'Due',
 	'detail.dueTime': 'Time',
+	'detail.duration': 'Length in minutes',
+	'detail.durationShort': 'min',
 	'detail.labels': 'Labels',
 	'detail.labelsPlaceholder': 'separated by commas',
 	'detail.project': 'Project',
@@ -742,6 +744,7 @@ export const en = {
 	'cal.settings': 'Settings',
 	'cal.noneChosen': 'None of the account’s calendars are chosen yet.',
 	'cal.allDay': 'All day',
+	'cal.resize': 'Drag to change the length',
 	'cal.openInGoogle': 'Open in Google Calendar',
 	'cal.beyondWindow':
 		'Events are kept from {from} to {to}. Outside that the grid shows only your own tasks.',