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.

scripts/screenshots.ts

@@ -356,7 +356,7 @@ await send('Page.enable')
 await send('Network.enable')
 // The agenda opens itself once a day; here it opens when asked.
 await send('Page.addScriptToEvaluateOnNewDocument', {
-  source: `localStorage.setItem('diary:agenda-seen', new Date().toDateString())`,
+  source: `localStorage.setItem('diary:agenda-seen', new Intl.DateTimeFormat('en-CA', { timeZone: 'Europe/Berlin' }).format(new Date()))`,
 })
 
 await viewport(1440, 900)