feat: add visual export functionality for dashboard analytics (#2712)

[desireddymohithreddy0925]

Description

Resolves #2712

This PR introduces a comprehensive Export Functionality designed to generate clean, shareable layouts for contribution analytics and developer insights. It consolidates the raw data downloads (CSV/JSON/Text PDF) and the new Visual exports (PNG/JPEG/Visual PDF) into an intuitive, unified Export Modal.

Changes Made

• Created ExportModal.tsx: A new hub for configuring and running both legacy exports and visual captures.
• Visual Capture Engine: Integrated html-to-image and jspdf to convert DOM nodes into high-quality images and properly formatted PDF layouts.
• Theme Support: Added a toggle to select the export theme (Current, Light, or Dark) ensuring portfolio-ready visuals regardless of the active UI mode.
• Widget Targeting: Tagged dashboard elements (dashboard-content, streak-tracker, repo-analytics, pr-metrics, activity-ring, coding-insights) with data-export-id attributes in src/app/dashboard/page.tsx for precise DOM targeting.
• Refactored ExportButton.tsx: Replaced the clustered inline buttons with a single "Export Analytics" trigger button that opens the unified modal.

Acceptance Criteria

• Allows users to export dashboard statistics and analytics as PDF.
• Allows users to export contribution heatmaps and charts as PNG/JPEG images.
• Allows users to select specific sections or widgets to include in the visual export.
• Supports both light and dark theme overriding for exports.
• Maintained functional parity with the original raw data exports.

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — 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

[Priyanshu-byte-coder]

Thanks for the contribution! We are currently in a code cleanup sprint and not accepting new features. Please see the discussion for details: #2651

This PR will be revisited once the sprint ends. In the meantime, contributions to testing, refactoring, bug fixes, or documentation are welcome.