Replace pytz with stdlib zoneinfo

[syssi]

Closes #1825

Summary

• Remove pytz dependency from pyproject.toml
• Replace pytz.timezone() with ZoneInfo() from stdlib zoneinfo module (Python 3.9+, now project minimum)
• Replace timezone.localize(dt) (pytz-specific) with dt.replace(tzinfo=timezone) (standard)
• Update BaseTzInfo type annotation to ZoneInfo

Tests

New tests in TestVacuum:

• test_timer_with_timezone — Timer parses cron and returns a timezone-aware next_schedule
• test_timer_disabled — Timer respects off state
• test_timer_ts — Timer creation timestamp is parsed from unix ID
• test_timer_ts_non_numeric_id — Non-numeric timer IDs (e.g. valetudo) return None for ts
• test_timer_list — vacuum.timer() builds Timer objects via ZoneInfo
• test_configure_wifi_with_timezone — UTC offset is 0.0 for UTC
• test_configure_wifi_with_offset_timezone — UTC offset is 8.0 for Asia/Shanghai

All 47 tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.49%. Comparing base (bacf59f) to head (b6710e8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2077      +/-   ##
==========================================
+ Coverage   83.29%   83.49%   +0.19%     
==========================================
  Files         199      199              
  Lines       19475    19519      +44     
  Branches     1058     1058              
==========================================
+ Hits        16222    16297      +75     
+ Misses       3071     3036      -35     
- Partials      182      186       +4     

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