feat: implement professional Export to PDF with optimized print media styles

[Dev1822]

Closes #197

Overview

This PR implements a robust, high-fidelity "Export to PDF" feature. It upgrades the simple "Print Preview" functionality to a polished, professional PDF export system. Using native browser print capabilities coupled with highly optimized print media stylesheets, it renders beautiful vector-scaled README documents without requiring heavy external dependencies.

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Key Enhancements

1. Theme-Lock & High Contrast Contrast

• Overrode and locked all :root markdown-rendering variables (--gh-preview-*) to light-mode values inside the print media query.
• The exported PDF will always print in standard GitHub light-theme aesthetics (black text, light-gray code block borders, soft backgrounds, blue links) even if the user is editing in Dark Mode.

2. Auto-Zoom Resolution

• Overrode the --preview-zoom variable to 1 !important for printing.
• Custom client-side zoom scales on the preview window are automatically ignored during export, guaranteeing that the generated PDF prints at precisely 100% scale.

3. Full-length Printing (Preventing Viewport Cutoffs)

• Set all parent container wrappers (html, body, #root, #app-builder, .main, .preview, .preview-body) to height: auto !important and overflow: visible !important.
• This ensures that long, multi-page READMEs print in their entirety instead of getting clipped at the active screen/viewport boundary.

4. Page Break Optimizations

• Applied break-inside: avoid / page-break-inside: avoid for code block wrappers (pre), blockquotes, tables, table rows (tr), list items (li), and images to prevent ugly horizontal splits.
• Applied break-after: avoid for headings (h1-h6) so headers stay anchored to the paragraphs below them.

5. Interface De-cluttering (Chrome Removal)

• Cleanly hides all layout elements including the main navbar, sidebars, dashboard headers, editor panel inputs, quality meters, back-to-top buttons, toasts, and overlays.

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UI Changes

• Added a new primary "Export to PDF" button to the main editor header right-side controls in ReadmeMaker.jsx with a clean SVG document icon.
• Upgraded the existing button in the preview panel PreviewPanel.jsx from "Print Preview" to "Export to PDF" with matching icon, styles, and notification toasts.

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Verification & Tests Done

• Successfully built the application using the production compiler (npm run build) to ensure zero esbuild or linting errors.
• Verified print layout previews in modern browsers:
  • Confirmed UI elements are hidden perfectly.
  • Confirmed light-mode values are locked and high-contrast is preserved.
  • Confirmed text scales cleanly and spans pages perfectly.