Fix timezone handling in date-based task filterin

[uncazzy]

Problem

Date-based task filtering functions (get_tasks_due_today, get_overdue_tasks, etc.) compare task due dates against UTC midnight boundaries. This causes tasks scheduled later in the day to be excluded from "due today" results if their UTC timestamp falls on a different calendar day.

Example: A task scheduled for 6:00 PM Eastern on December 2nd is stored as 2025-12-03T00:30:00Z in UTC. When checking "tasks due today" on December 2nd, the function would exclude this task because its UTC date is December 3rd.

Solution

• Add timezone parameter to all date-based functions (defaults to America/New_York)
• Calculate proper day boundaries by converting local start/end of day to UTC
• Add tzdata dependency for cross-platform timezone support (required on Windows)

Affected Functions

• get_tasks_due_today()
• get_tasks_due_tomorrow()
• get_tasks_due_in_days()
• get_overdue_tasks()
• get_tasks_due_this_week()
• get_engaged_tasks()
• get_next_tasks()

Usage

Works out of the box with Eastern Time default:

get_tasks_due_today()

Or specify a different timezone:

get_tasks_due_today(timezone="Europe/London")