[Enhancement] add due date feature to todos

[niat-web]

Feature Request

What:
Allow users to optionally set a due date
for each todo item.

Expected behavior:
Creating todo:

• Optional date picker field "Due Date"
• If not set, no due date shown on card

Todo card:

• Show due date below description
• Format: "Due: Jul 15, 2026"
• Overdue todos (past due date) shown in red
• Today's todos shown in orange

Sorting:

• Option to sort todos by due date

Database changes needed:
Run this in Supabase SQL Editor:
ALTER TABLE todos ADD COLUMN due_date TIMESTAMP WITH TIME ZONE;

Acceptance Criteria:

• Due date field in create todo form
• Due date shown on todo card
• Overdue todos highlighted in red
• Today due todos highlighted in orange
• Database column added