feat: add task due-date filtering and overdue indicators

[baskard]

Problem / motivation

Users currently have no way to filter tasks by due date or quickly see which tasks are overdue. This leads to missed deadlines and poor prioritization.

Proposed solution

• Add a "Due soon" and "Overdue" filter on the Dashboard
• Show a red badge on task cards when the due date has passed
• Add a "due this week" quick-filter button

Acceptance criteria

• Tasks with past due dates show a red "Overdue" badge
• Tasks due within 7 days show an amber "Due soon" badge
• Dashboard has filter tabs: All | Due Soon | Overdue
• Filtering persists across page refreshes (localStorage)
• Backend GET /api/tasks supports ?filter=overdue and ?filter=due_soon query params

Additional context

Priority: Medium. Impacts daily active users who manage time-sensitive projects.