[Feature] Implement Task Dependencies with Blocking/Blocked-By Relationships

[knoxiboy]

Description
Tasks in DevTrack are independent — there is no way to express that Task B cannot start until Task A is complete. Add dependency relationships (blocks, blocked-by) with visual indicators on the Kanban board.

Motivation
Task dependencies are essential for project planning. Without them, team members may start work on tasks whose prerequisites aren't yet complete, leading to wasted effort and rework.

Implementation

1. Add a task_dependencies table in Supabase: blocker_task_id, blocked_task_id.
2. Show a dependency badge on task cards (e.g., "Blocked by DT-45").
3. Prevent moving a blocked task to "In Progress" until its blockers are resolved.
4. Add a simple dependency graph visualization on the task detail page.

Level: Advanced | Affected Files: src/, supabase/

[knoxiboy]

/assign gssoc'26
@Priyanshu-byte-coder Please assigne this issue to me.