fix(alarm): prevent duplicate restored triggers

[Zoupers]

Summary

• bind restored alarm tasks to their own IDs, delays, messages, and repeat settings so multiple restored alarms do not all fire the final entry
• make alarm restoration idempotent for an already scheduled alarm ID
• normalize naive and timezone-aware trigger timestamps to the runtime timezone, including UTC timestamps sent by the admin UI
• persist timezone offsets and replace the alarm state file atomically

Validation

• uv run --frozen pytest tests/unit/test_alarm_tools.py tests/unit/test_i18n.py tests/unit/test_admin_api.py::test_admin_alarm_accepts_browser_utc_timestamp -q — 52 passed
• uv run --frozen ruff check src/coworker/tools/alarm_tools.py tests/unit/test_alarm_tools.py tests/unit/test_admin_api.py — passed
• uv run --frozen mypy src/coworker/tools/alarm_tools.py — passed
• original two-overdue-alarm reproduction now emits one event for each alarm and leaves no stale persisted entries
• uv run --frozen pytest tests/unit -q — 1448 passed, with 2 unrelated existing failures:
  • tests/unit/test_admin_api.py::test_config_patch_explicitly_clears_one_override
  • tests/unit/test_api.py::test_api_defaults_bind_locally_and_require_desktop_authentication

Risks and compatibility

• Existing persisted timestamps without an offset remain supported and are interpreted in the runtime's local timezone.
• Newly persisted and API-listed timestamps include an explicit timezone offset.
• This change does not introduce durable inbox acknowledgements; delivery across a hard process crash still follows the existing in-memory inbox semantics.