cli: add optional due_tz field for timezone-aware due dates (#337)

* feat: add optional due_tz field for timezone-aware due dates

Due dates can now optionally carry an IANA timezone (e.g. America/New_York).
When set, the wall-clock time in due_at is interpreted in that timezone for
overdue checks, "due soon" detection, and display formatting. Floating
(timezone-less) tasks remain the default and behave identically to before.

Key changes:
- New nullable due_tz column (migration v8) and field across all layers
- nextOccurrence refactored to UTC-based Date methods, making recurrence
  arithmetic timezone-independent and fixing a latent DST drift bug
- CLI: --tz flag on add/update commands, shell completions
- Display: zoned tasks show timezone abbreviation (e.g. "2026-03-20 15:00 EST")
- filterByDue: "today" computed in task's timezone for zoned tasks

* cli: bump version to 0.0.6

Merge pull request #333 from tchayen/fix-publish

cli: bump version to 0.0.4 (#332)

cli: bump version to 0.0.3 (#330)