fix(agenda): fix cache, event visibility, and recurring event handling

[stvsu]

Summary

Fixes several bugs in the agenda extension discovered through real-world use with iCal feeds containing recurring events, multi-day all-day events, and non-UTC timezones.

Cache reliability

• eventCalendars was a useRef — changes didn't trigger re-renders, so calendar name tags on events were stale after a refresh. Changed to useState.
• Cache never showed last refresh time — loadFromCache wasn't returning the cache timestamp, so the smart refresh timer couldn't align to the last fetch. Fixed.
• Cache strategy: replaced the hard 5-minute TTL with a background refresh model — the cache is shown immediately on open, then a network refresh runs on the user's configured interval. This avoids a blank screen on every open when the cache has expired.
• Stale events shown from cache — on load, the cached event list is now filtered through isFutureEvent so events that ended since the cache was saved don't briefly appear before the network refresh completes.

Event visibility

• isAllDayEvent only recognised 24-hour events — multi-day all-day events (e.g. a 3-day conference) were not identified as all-day. Fixed by checking midnight boundaries rather than exact 24h duration.
• isFutureEvent used start time — in-progress events (meeting started 10 mins ago, ends in 50 mins) were filtered out. Now uses end time for timed events and end date for all-day events.
• Multi-day all-day events only appeared on their start date — groupEventsByDate now expands them across every date they span.
• Past date rows shown for ongoing multi-day events — a 4-day event starting Monday would show Monday/Tuesday rows on Wednesday. Fixed with a render-time filter on date keys.

Recurring events

• RECURRENCE-ID overrides were ignored — when a single occurrence of a recurring event is rescheduled, the original time was shown instead of the new time. The rrule expansion loop now checks item.recurrences for each occurrence and uses the override if present.
• In-progress recurring occurrences not shown — rrule.between() was called with rangeStart = new Date(), so occurrences that started before the current moment were excluded even if the event was still running. Fixed by using midnight today as rangeStart with an isFutureEvent guard to filter out occurrences that have already ended.
• Rescheduled occurrences with past original dates missing — if an occurrence's original date falls before rangeStart, rrule.between() never returns it, so the override was never processed. Added a secondary sweep of item.recurrences to catch these.

UX

• All-day events show "All Day" subtitle instead of empty string
• Navigation title shows last refresh time
• Refresh action (using Keyboard.Shortcut.Common.Refresh) available in all list states

Test plan

• Recurring events show correctly throughout the day, including in-progress occurrences
• Rescheduled recurring occurrences show at the new time, not the original
• Multi-day all-day events appear on each day they span
• Only today-and-future date rows are shown (no stale past rows for ongoing multi-day events)
• Opening the extension shows cached events immediately with no stale ended events
• Refresh interval setting is respected; refresh timer aligns to last fetch, not extension open time
• npm test passes (119 tests)