feat(milestones): add milestone tracking with progress visualization

[desireddymohithreddy0925]

Summary

Implements a complete milestone tracking system, allowing users to group tasks into milestones and visualize their progress dynamically.

Closes #2886

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing functionality)
• 📝 Documentation update
• 🎨 Code style update (formatting, renaming)
• ♻️ Refactoring (no functional changes)
• 🧪 Testing
• 🏗️ Build / CI/CD / Infrastructure

Description

This PR addresses issue #2886 by adding a fully functional ProjectMilestones component alongside a robust set of backend APIs.

Changes made:

• Database Schema: Replaced the previous partial implementation with the accurate relational model specified in the issue: a milestones table containing (id, name, dueDate) and a tasks table containing (id, title, completed, milestone_id).
• Backend APIs:
  • GET/POST /api/milestones: Implemented creation and optimized data fetching, including aggregations to calculate completedTasks, totalTasks, and array building for taskIds via SQL/Supabase logic.
  • PUT/DELETE /api/milestones/[id]: For managing milestones and unlinking/re-linking task updates.
  • GET/POST/PUT/DELETE /api/tasks: For granular task creation and completion status updates.
• Frontend Widget: Entirely refactored ProjectMilestones.tsx from using local-storage-mocking to connecting with our real data backend. It includes real-time progress calculations (Math.round((completedTasks / totalTasks) * 100)), UI to add tasks sequentially, toggle completion, and delete entities optimistically.

Testing

• Created, updated, and deleted milestones via the UI.
• Tested adding tasks to individual milestones and toggling completed status to verify percentage accuracy.
• Confirmed database integration logic properly enforces RLS policies for users accessing only their own entities.
• Executed local type checks (pnpm typecheck without any errors).

Screenshots / Screen Recording

(Attach a screenshot of the Project Milestones widget with the progress bar in action)

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus