feat: Calendar — the next event on Home, and a full add/edit lifecycle

[eddiegulay]

Adds Google Calendar support to Tempo, via the device's CalendarContract provider — the same one the Google Calendar sync adapter writes into, so the launcher shows the very agenda the user's other devices show. No OAuth, no network, no Play Services.

What's in it

• Home's top-right cluster shows the next event, set vertically (縦書き), replacing the Japanese date. Kanji and kana stack upright; a run of Latin turns a quarter-turn and reads as one stroke; short numbers sit upright (縦中横). Falls back to the date when there's nothing ahead or no access. Tapping it is the only way into the agenda.
• The 予定 page — the next fortnight, grouped by day, in-progress events marked いま.
• Add and edit in place, on a 栞 wheel rather than a Material picker.
• Recurring events and read-only calendars are view-only, and hand off to the system calendar app. Editing one occurrence of a series means writing the provider's exception rows, and getting it subtly wrong silently rewrites a meeting for everyone on the invite — so Tempo doesn't try.
• Every mutation is confirmed, restating the event and its consequence. A calendar is shared state.

Reliability

The lifecycle now reports and recovers rather than failing silently:

• Loading, empty and failed are three distinct states. An unreadable calendar used to render as 予定はありません — a free day and a broken one were the same picture, and the user would have believed it.
• A failed write keeps the user in the composer with their draft intact and the reason stated, instead of returning to an agenda that doesn't contain the event they just typed.
• Every fault names itself and offers the one thing that fixes it: ask for access again, open Settings, add an account, or retry.

Permissions

Adds READ_CALENDAR and WRITE_CALENDAR, requested when the page is opened, never at launch.

Verification

Built, unit-tested (30 passing, incl. the vertical-text segmentation and the fault-recovery contract), and driven end-to-end on an emulator — including revoking WRITE_CALENDAR mid-session to confirm the save fails visibly, the draft survives, and the retry writes through to the provider.