Fix #195: Add Vector PDF export and modal options

[ThisIs-Developer]

Fixes #195

Root Cause:
The current PDF export architecture uses \html2canvas\ and \jsPDF\ to capture the entire rendered markdown document as a large raster image. This image is then sliced and placed into a PDF document. Because it is rasterized, text in the PDF is unselectable, fuzzy, and appears pixelated or blurred when zoomed in.

Implementation Approach:
To fix this while keeping the legacy raster export safe, I've split the PDF export options:

1. PDF (Vector): This replaces the primary 'PDF' export button and invokes native browser printing (\window.print()). A new @media print\ CSS block was added to \styles.css\ that hides the editor pane, header, and toolbars, and makes the preview pane take up the full print document. This outputs a true vector PDF with crisp, selectable text.
2. PDF (Legacy Image): The old \html2canvas\ + \jsPDF\ logic was preserved under a new 'Legacy PDF' dropdown option. It acts as a fallback for browsers where printing behaves poorly or if users specifically want a raster image inside a PDF wrapper.

Before/After Behavior:

• Before: Exporting a PDF resulted in a large file containing a single raster image where text was blurry.
• After: Exporting a PDF via the new default method opens the native print dialog, generating a fast, vector-based PDF with perfectly crisp and selectable text. Diagrams and MathJax still render correctly inside the preview block prior to printing.

Testing Performed:

• Added @media print\ styles.
• Connected native \window.print()\ to the existing \exportPdf\ click listener.
• Ensured \exportPdfRaster\ logic retains the older complex multi-pass rendering.

Limitations or trade-offs:

• The new vector PDF relies on browser-native pagination during printing instead of manual height calculation. This is generally more robust for text but might occasionally page-break awkwardly inside large code blocks depending on browser heuristics.

Confirmation: Unrelated export features (HTML export, PNG export, Markdown preview) were not broken.

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