fix: reset weekly goal progress on new week boundary

[aisshwaryaa8-collab]

Summary

Weekly goals were not resetting their progress when a new week began.
This fix adds a week_start column to the goals table and updates the
API route to reset current to 0 when the stored week differs from the
current ISO week.

Closes #2390

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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 behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• supabase/schema.sql: added week_start DATE column to the goals table
• src/app/api/goals/route.ts: added week_start to the update payload when a weekly reset occurs
• src/components/GoalTracker.tsx: added week_start to the Goal interface and displays the active week range for weekly goals

How to Test

1. Create a weekly goal (e.g. "20 commits this week")
2. In Supabase SQL Editor, manually set week_start to a past date:
   UPDATE goals SET week_start = '2026-06-01', current = 5 WHERE id = '';
3. Reload the dashboard and check the Goals widget

Expected result:

current resets to 0 and week_start updates to the
current week's Monday. The week range label (e.g. "Jun 9 – Jun 15")
appears below the goal title for weekly goals.

Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

[github-actions]

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[aisshwaryaa8-collab]

@Priyanshu-byte-coder Please approve and merge this pr