feat: support absolute datetime for reset timers

[fanyu]

Summary

• add a date mode (d) to both Block Reset Timer and Weekly Reset Timer
• in date mode, render the reset point as an absolute UTC timestamp instead of a duration
• keep existing duration/progress modes unchanged and gracefully fall back to duration if reset timestamp is unavailable

UX

• p: cycle time/full bar/short bar (existing)
• s: compact formatting for duration/date (existing behavior extended to date mode)
• d: toggle absolute datetime mode (new)

Implementation

• add formatUsageResetAt(resetAt, compact) utility
• add shared metadata helpers isUsageDateMode / toggleUsageDateMode
• wire date mode into both reset timer widgets and editor modifier text
• update README docs for widget behavior and keybinds
• add/extend tests for date mode rendering and reset timestamp formatter

Verification

• npx tsc --noEmit && npx eslint . --config eslint.config.js --max-warnings=0
• npx vitest run src/utils/__tests__/usage.test.ts src/widgets/__tests__/BlockResetTimer.test.ts src/widgets/__tests__/WeeklyResetTimer.test.ts