Fix month/year range parsing overflow on month-end dates

[arran4]

Motivation

• TimeParser.parseRange produced incorrect start dates for month/year ranges when the current day did not exist in the target month (e.g., subtracting one month from Mar 31), causing overflows instead of clamping to the last valid day.
• The code needed to preserve time-of-day while safely handling month and year rollovers and leap-day behavior.
• An edge-case regression test was added to capture these month-end and leap-year scenarios.

Description

• Added helper functions _subtractMonths, _subtractYears, and _daysInMonth to lib/utils/time_parser.dart to centralize correct date arithmetic and clamp day-of-month to the last valid day of the target month.
• Replaced ad-hoc DateTime arithmetic in both the duration-based (last N month/year) and phrase-based (last month, last year) branches of parseRange to use the new helpers and preserve time components.
• Added an edge-case test should clamp day for month and year range phrases in test/time_parser_test.dart that verifies correct clamping for last 1 month and last 1 year with preserved time-of-day.

Testing

• Attempted dart format . but it failed in this environment because dart is not installed, so formatting could not be verified here.
• Attempted flutter analyze but it failed in this environment because flutter is not installed, so static analysis could not be run here.
• Attempted dart test . but it failed in this environment because dart is not installed, so the new unit test was not executed here; CI should run the test suite and is expected to pass with the changes.

---

Codex Task