fix(#370): allow modifiers after rounding in time_period_from/to

[MindFreeze]

Fixes #370.

The calculateTimePeriod parser previously rejected Grafana-style anchors with a modifier after a rounding operator (e.g. now/d+7h, now-1d/d+23h), even though the README claims to follow Grafana's time range format. The parser now applies offset and rounding tokens left-to-right, unblocking windows like "yesterday 23:00 to today 07:00".

Summary

• Refactored parseTimeString in src/utils.ts from a single-shot regex into an iterative tokenizer that walks offsets ([+-]N{unit}) and roundings (/{unit}) left-to-right. No new dependencies.
• Added __tests__/calculateTimePeriod.test.ts (none existed) with deterministic time via jest.setSystemTime, covering the existing patterns, the new ones (now/d+7h, now-1d/d+23h, now/d-1h), the quiet-hours combo, and invalid-input rejection.
• Documented the new examples and a "Quiet hours (23:00–07:00)" YAML snippet in the Time Period section of the README.

Examples now supported

Input	Meaning
now/d+7h	Today at 07:00
now-1d/d+23h	Yesterday at 23:00
now/d-1h	Yesterday at 23:00 (equivalent)

Test plan

• npm run lint — no new warnings/errors
• npm test — 114/114 tests pass across 9 suites, including the new calculateTimePeriod suite
• Manual: configure a card with time_period_from: "now-1d/d+23h" and time_period_to: "now/d+7h" and confirm the 8-hour window renders correctly
• Manual: re-test existing patterns (now-7d, now-1d/d → now/d) to confirm no regression