feat: implement PDF generation and update Dockerfile

- Add PDF generation using go-pdf/fpdf (Pure Go library)
- Walk markdown AST and render directly to PDF
- Update Dockerfile to include themes directory and config
- Fix ENTRYPOINT format for serve mode

Co-Authored-By: Claude (glm-5) <noreply@anthropic.com>

go.sum

@@ -1,5 +1,7 @@
 github.com/fsnotify/fsnotify v1.9.0 h1:2Ml+OJNzbYCTzsxtv8vKSFD9PbJjmhYF14k/jKC7S9k=
 github.com/fsnotify/fsnotify v1.9.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
+github.com/go-pdf/fpdf v0.9.0 h1:PPvSaUuo1iMi9KkaAn90NuKi+P4gwMedWPHhj8YlJQw=
+github.com/go-pdf/fpdf v0.9.0/go.mod h1:oO8N111TkmKb9D7VvWGLvLJlaZUQVPM+6V42pp3iV4Y=
 github.com/gorilla/mux v1.8.1 h1:TuBL49tXwgrFYWhqrNgrUNEY92u81SPhu7sTdzQEiWY=
 github.com/gorilla/mux v1.8.1/go.mod h1:AKf9I4AEqPTmMytcMc0KkNouC66V3BtZ4qD5fmWSiMQ=
 github.com/yuin/goldmark v1.7.16 h1:n+CJdUxaFMiDUNnWC3dMWCIQJSkxH4uz3ZwQBkAlVNE=