Removes the `unipdf` and `rmapi` library dependencies

[joagonca]

Summary

Removes unipdf and rmapi dependencies (licensing issues) and replaces with Cairo-based PDF rendering.

Key Changes

• Removed: unipdf, juruen/rmapi
• Added: go-cairo, pdfcpu
• New: Internal encoding/rm and archive packages (copied from AGPL rmapi fork)
• Renamed: RenderRmapi() → RenderPDF()

Requirements

Cairo must be installed for building and running:

• Debian/Ubuntu: apt-get install libcairo2-dev pkg-config
• macOS: brew install cairo pkg-config
• Docker: Now uses debian:bookworm-slim base (includes Cairo runtime)

Breaking Changes

• Requires CGO (no more CGO_ENABLED=0 builds)
• Docker base image changed from scratch to debian:bookworm-slim

All reMarkable PDF rendering features preserved (annotations, highlighters, v3/v5 formats, etc).

[joagonca]

@ddvk I've done a bit of copying/pasting, and I've made the build process similar/equal to the one suggested in the v6_rmc branch.

Though I don't have access to v5 content to test. That's why I've opened a draft PR. Any suggestions?

[rmitchellscott]

Isn't rmapi also AGPL 3.0?

[joagonca]

@ddvk Is this OK? If so, I could bump the v6_rmc @ #392 branch to accommodate these changes. Currently there are a few conflicts due to the rendering and the modules.