feat(export): implement CSV export for task data

[desireddymohithreddy0925]

Summary

Adds an Export CSV button to the Kanban Board to easily share, backup, or analyze tasks in spreadsheets.

Closes #2887

---

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

---

What Changed

• Added tasksToCSV and downloadCSV to src/lib/export-utils.ts to format task arrays securely (accounting for commas and quotes).
• Integrated the download trigger button seamlessly into the Kanban board header.

---

How to Test

1. Navigate to the dashboard and locate the Kanban Board widget.
2. Ensure you have at least one active task created.
3. Click the "Export CSV" button in the board's header.
4. Open the downloaded .csv file in Excel or Numbers.

Expected result: The file downloads successfully and contains id, title, status, priority, dueDate, createdAt, and tags columns, accurately matching the active UI tasks.

---

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]

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