docs: add interactive documentation system with Go+HTMX

- Add internal/docs package with markdown renderer (goldmark)
- Create docs layout template with sidebar navigation
- Implement hierarchical navigation auto-generated from docs folder
- Add table of contents generator (extract ## headings)
- Add syntax highlighting for code blocks (highlight.js)
- Add mobile responsive design
- Add /docs routes to main.go

The documentation system features:
- Dark theme matching app design
- Collapsible sidebar sections (Getting Started, User Guide, Device Setup, API Reference, Contributing)
- Table of contents for each page
- Breadcrumb navigation
- Full-text search (client-side JavaScript, API endpoint ready)
- Syntax highlighting for code blocks
- Mobile-friendly with hamburger menu

All documentation is served from /docs route, no authentication required.
Markdown files are rendered using goldmark with GFM extensions and syntax highlighting.

go.sum

@@ -2,10 +2,14 @@ github.com/ArcadiaLin/go-epub v0.1.1 h1:13roe62tarrZ1Y1QTxE+Bzd/NKlChhRzegLVmU5H
 github.com/ArcadiaLin/go-epub v0.1.1/go.mod h1:GY09AG6jnEbsYytkw6VeICLOt25F1GQDGXjYS7cxNhU=
 github.com/a-h/templ v0.3.977 h1:kiKAPXTZE2Iaf8JbtM21r54A8bCNsncrfnokZZSrSDg=
 github.com/a-h/templ v0.3.977/go.mod h1:oCZcnKRf5jjsGpf2yELzQfodLphd2mwecwG4Crk5HBo=
+github.com/alecthomas/chroma v0.10.0 h1:7XDcGkCQopCNKjZHfYrNLraA+M7e0fMiJ/Mfikbfjek=
+github.com/alecthomas/chroma v0.10.0/go.mod h1:jtJATyUxlIORhUOFNA9NZDWGAQ8wpxQQqNSB4rjA/1s=
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/dlclark/regexp2 v1.4.0 h1:F1rxgk7p4uKjwIQxBs9oAXe5CqrXlCduYEJvrF4u93E=
+github.com/dlclark/regexp2 v1.4.0/go.mod h1:2pZnwuY/m+8K6iRw6wQdMtk+rH5tNGR1i55kozfMjCc=
 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/gabriel-vasile/mimetype v1.4.12 h1:e9hWvmLYvtp846tLHam2o++qitpguFiYCKbn0w9jyqw=
@@ -65,6 +69,11 @@ github.com/valyala/bytebufferpool v1.0.0 h1:GqA5TC/0021Y/b9FG4Oi9Mr3q7XYx6Kllzaw
 github.com/valyala/bytebufferpool v1.0.0/go.mod h1:6bBcMArwyJ5K/AmCkWv1jt77kVWyCJ6HpOuEn7z0Csc=
 github.com/valyala/fasttemplate v1.2.2 h1:lxLXG0uE3Qnshl9QyaK6XJxMXlQZELvChBOCmQD0Loo=
 github.com/valyala/fasttemplate v1.2.2/go.mod h1:KHLXt3tVN2HBp8eijSv/kGJopbvo7S+qRAEEKiv+SiQ=
+github.com/yuin/goldmark v1.4.5/go.mod h1:rmuwmfZ0+bvzB24eSC//bk1R1Zp3hM0OXYv/G2LIilg=
+github.com/yuin/goldmark v1.7.16 h1:n+CJdUxaFMiDUNnWC3dMWCIQJSkxH4uz3ZwQBkAlVNE=
+github.com/yuin/goldmark v1.7.16/go.mod h1:ip/1k0VRfGynBgxOz0yCqHrbZXhcjxyuS66Brc7iBKg=
+github.com/yuin/goldmark-highlighting v0.0.0-20220208100518-594be1970594 h1:yHfZyN55+5dp1wG7wDKv8HQ044moxkyGq12KFFMFDxg=
+github.com/yuin/goldmark-highlighting v0.0.0-20220208100518-594be1970594/go.mod h1:U9ihbh+1ZN7fR5Se3daSPoz1CGF9IYtSvWwVQtnzGHU=
 golang.org/x/crypto v0.46.0 h1:cKRW/pmt1pKAfetfu+RCEvjvZkA9RimPbh7bhFjGVBU=
 golang.org/x/crypto v0.46.0/go.mod h1:Evb/oLKmMraqjZ2iQTwDwvCtJkczlDuTmdJXoZVzqU0=
 golang.org/x/net v0.47.0 h1:Mx+4dIFzqraBXUugkia1OOvlD6LemFo1ALMHjrXDOhY=