agenda: today is the workspace's day, not the browser's

The browser grouped the agenda by its own midnight and remembered that it had
opened it by its own date, while the server counts due dates, mails and the
diary in `WORKSPACE_TIMEZONE`. A laptop on a trip, or a browser set to UTC,
showed a card as late that the morning mail called due today, and opened the
agenda a second time on one workspace day or not at all. A day was also taken
to be 86400 seconds, which it is not when the clocks change.

The agenda answer now carries `today`: the date in the configured timezone,
when it starts and ends, and when the seventh day after it ends, all from the
same `day_bounds` the mails use. The browser groups by those and keys the
once-a-day opening on that date. The screenshot run marks the agenda as seen
under the same date.

web/src/lib/api.ts

@@ -194,6 +194,8 @@ export interface Agenda {
   diary_today: boolean | null
   /** Diary entries from this day in earlier years, the latest year first. */
   on_this_day: DiaryEntry[]
+  /** Today in the workspace's timezone, which is what due dates are counted in. */
+  today: { date: string; start: number; end: number; week_end: number } | null
 }
 
 /** A diary entry, by the local day it is about. */

web/src/lib/store.svelte.ts

@@ -299,7 +299,7 @@ class Workspace {
    * What is due or in hand on every board, kept loaded all the time: the bar
    * counts from it, so what is late is in sight without going to look.
    */
-  agenda = $state<Agenda>({ cards: [], diary_today: null, on_this_day: [] })
+  agenda = $state<Agenda>({ cards: [], diary_today: null, on_this_day: [], today: null })
   agendaCursor = $state(0)
 
   /** `C`: the day the calendar's cursor is on, `2026-10-20`, and its month's entries. */
@@ -386,16 +386,18 @@ class Workspace {
     return this.list?.cards[this.cardCursor]
   }
 
-  /** The agenda in the order it is drawn and walked, bucketed by urgency. */
+  /**
+   * The agenda in the order it is drawn and walked, bucketed by urgency. The
+   * day is the server's, in the workspace's timezone, so a browser on a trip
+   * groups the cards the way the morning mail does.
+   */
   get agendaGroups(): { key: 'overdue' | 'today' | 'week' | 'doing'; cards: AgendaCard[] }[] {
-    const midnight = new Date()
-    midnight.setHours(0, 0, 0, 0)
-    const start = Math.floor(midnight.getTime() / 1000)
+    const { start, end, week_end } = this.agenda.today ?? { start: 0, end: 0, week_end: 0 }
     const groups = { overdue: [] as AgendaCard[], today: [] as AgendaCard[], week: [] as AgendaCard[], doing: [] as AgendaCard[] }
     for (const card of this.agenda.cards) {
       if (card.due_at !== null && card.due_at < start) groups.overdue.push(card)
-      else if (card.due_at !== null && card.due_at < start + 86400) groups.today.push(card)
-      else if (card.due_at !== null && card.due_at < start + 8 * 86400) groups.week.push(card)
+      else if (card.due_at !== null && card.due_at < end) groups.today.push(card)
+      else if (card.due_at !== null && card.due_at < week_end) groups.week.push(card)
       else if (card.role === 'doing') groups.doing.push(card)
     }
     return (['overdue', 'today', 'week', 'doing'] as const).map((key) => ({ key, cards: groups[key] }))
@@ -1306,8 +1308,8 @@ class Workspace {
    * point is to be told, not to have to remember to ask.
    */
   async openAgendaIfDue() {
-    const today = new Date().toDateString()
-    if (localStorage.getItem('diary:agenda-seen') === today) return
+    const today = this.agenda.today?.date
+    if (!today || localStorage.getItem('diary:agenda-seen') === today) return
     const { late, today: due } = this.dueCounts
     if (late + due === 0) return
     localStorage.setItem('diary:agenda-seen', today)