Upgrade PDF export to high-resolution vector PDF

[weiykong]

Replaced rasterized CGImage PDF drawing with native ImageRenderer vector PDF drawing. This enables sharp, infinite-resolution (SVG-quality) PDF outputs with fully selectable and searchable text, while preserving proper pagination/slicing for blocks spanning across multiple pages.

[b451c]

Merged - thank you, this is a genuinely valuable upgrade!

I verified the output on a fixture with a ~120-line code block spanning multiple pages: 4 pages, fully selectable and searchable text, fonts embedded as subsets, every line present and in order across page breaks, and the file came out smaller than the old rasterized output (25 KB). The clip/translate math for slicing oversized blocks checks out, and behavior at page boundaries matches the previous renderer.

One note: the PR also wrapped two #Preview blocks in #if !STRIP_PREVIEWS guards - that flag is not defined anywhere in this project, so I removed those two hunks in a small cleanup commit on main right after the merge (cd85005). No behavior change either way, just keeping the tree free of undefined build flags. If STRIP_PREVIEWS comes from your local tooling, no worries - everything else landed as-is.

This will ship in the next release. Thanks again!

[b451c]

v1.7.0 with this change is now live: https://github.com/b451c/quickmd/releases/tag/v1.7.0 - thanks again!