feat: Add Raw/Preview Toggle to Markdown Export Panel

[iRAFEEK]

Description

Adds a Raw / Preview toggle to the Markdown tab of the Export Panel, so users can verify the badge image renders correctly before copying the snippet to their GitHub README.

Fixes #7813

Note on the existing Live Preview: The page already has a Live Preview widget in the customization studio. The visual output is largely the same — both show the badge for your username. This toggle is a UX convenience scoped to the Export Panel itself: it lets users confirm the exact Markdown snippet URL renders correctly as a plain <img> (the way GitHub displays it in a README) without having to scroll back up to the Live Preview section.

Changes

• Added viewMode state ('raw' | 'rendered') to ExportPanel.tsx
• Rendered a pill-style Raw / Preview toggle button group, visible only when the Markdown tab is active
• In Preview mode: extracts the image URL from the markdown snippet and renders it as <img>; falls back to a placeholder message if no username is set
• Resets to Raw mode when switching export format tabs

Pillar

• 🎨Pillar 1 — New Theme Design
• 📐Pillar 2 — Geometric SVG Improvement
• 🕐Pillar 3 — Timezone Logic Optimization
• ⚡Other (Bug fix, refactoring, docs)

Visual Preview

Checklist before requesting a review:

• I have performed a self-review of my code
• I have tested the changes locally
• My changes follow the project's existing code style

[retenta-bot]

This PR adds a Raw/Preview toggle to the Markdown export tab, which is a great UX improvement for users verifying badge image rendering. It aligns with previous decisions made regarding enhancing user experience in the Export Panel. Please ensure to review the implementation thoroughly. Thank you for your contribution!

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