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.

docs/wiki/API.md

@@ -20,7 +20,7 @@ sealed or not.
 | `DELETE` | `/api/trash/{id}` | delete one thing in the trash for good |
 | `POST` | `/api/trash/{id}/restore` | back where it was deleted from, or to its space when that folder is gone |
 | `GET` | `/api/spaces/{id}/board` | the lists and cards of one space |
-| `GET` | `/api/agenda` | open cards due within a week or in hand, on every board, whether the diary has today, and its entries `on_this_day` in earlier years |
+| `GET` | `/api/agenda` | open cards due within a week or in hand, on every board, whether the diary has today, its entries `on_this_day` in earlier years, and `today` in `WORKSPACE_TIMEZONE`, with its bounds |
 | `GET` | `/api/diary/days?month=2026-10` | the diary's entries in a month, each with its local `day` |
 | `GET` | `/api/diary/around/{id}` | the entries before and after a diary entry, by date |
 | `GET` | `/api/nodes/{id}/templates` | the templates of the space the node is in |

docs/wiki/Boards-and-Agenda.md

@@ -107,7 +107,9 @@ does not make a second copy. The board and the agenda show the rule as
 | `T` | today's diary entry |
 | `t`, `q`, `Esc` | close |
 
-The agenda opens by itself once a day when something is late or due. The bar
+The agenda opens by itself once a day when something is late or due. "Today"
+is the day in `WORKSPACE_TIMEZONE`, the same as for the mails, so a browser in
+another timezone groups the cards the same way. The bar
 shows the `late` and `today` counts whenever either is above zero. With mail
 set up, cards due today are mailed in the morning too; see
 [Backup and mail](Backup-and-Mail.md#mail).