Feat/milestone planner

[Bhavy12-cell]

Closes #2197

Added a Development Milestone Planner widget to the dashboard.

Features:

• Create custom milestones with title, category, target value and date
• Track progress with visual progress bar
• Status indicators: Completed, On Track, At Risk, Behind
• Progress forecasting based on current activity rate
• +1 increment button for manual progress tracking
• Categories: Commits, Streak Days, Projects, Custom
• Expand/collapse for milestone details
• Persistent storage using localStorage
• Fully responsive layout

[vercel]

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[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