fix: re-apply the theme after missed boundaries

[clappingmonkey]

What & why

Fixes cases where the theme didn't switch, or switched late (audit item H3):

• Sleep across two boundaries: the period on wake matches the one before sleep, so a manual /theme from before sleep was kept, e.g. sleeping from evening until the next evening.
• Long checkIntervalMs: switches waited for the next tick. At 24h the theme never switched, and at 13h some periods were skipped entirely.

Changes:

• New exported pure helper nextPeriodChange(from, cfg, horizonMs). It returns the exact next day/night change by checking only the instants where one can happen: 15-minute UTC marks in fixed mode, and each solar day's start, sunrise and sunset in solar mode. So even very short periods near the polar circle are found (for example a 2-minute night at 66.34°N).
• tui remembers when it last checked the period. It re-applies the theme if any boundary was crossed since then, even when the period now looks unchanged (looking back up to 400 days).
• A one-shot timer fires at the next boundary, so switches happen on time whatever checkIntervalMs is. The interval stays as a fallback. The timer is cleared on abort/dispose.
• Failed or not-ready applies still don't advance state, so they are retried.
• README: "How it behaves" and the checkIntervalMs row are updated (this also fixes the README part of audit item L5).

Behavior impact

• Theme-switching logic (periodFor, boundaries, gentle override)
• Plugin options (dayTheme, nightTheme, dayStartHour, nightStartHour,
  checkIntervalMs, toast) — README options table updated
• Lifecycle / timer / toast behavior
• Packaging (package.json exports, files, published contents)
• No user-facing behavior change

After waking up past a boundary, a manual /theme is now replaced by the configured theme for the current period. Within a period it is still kept.

Testing

• npm run typecheck passes
• npm test passes: 56 tests (11 new nextPeriodChange unit tests, 7 new tui tests). Also passes under TZ set to UTC, Europe/London, America/St_Johns, Australia/Lord_Howe, Asia/Kathmandu, Pacific/Chatham and America/New_York.
• Manually verified in opencode (if behavior changed): not done

Extra checks:

• Brute-force comparison of nextPeriodChange against a 30-second scan for 246 configs over all of 2026: 0 mismatches.
• Mutation checks: removing the missed-boundary check or the boundary timer, or advancing state on a failed apply, each fails a test.

Checklist

• PR title follows Conventional Commits
  (enforced by the PR Title check; drives the release-please version bump)
• Defaults still work with zero config
• Docs updated if behavior/config changed
• No secrets or credentials introduced