Add TypeScript migration, image resizing, media upload UX, and multimedia support

- Migrate static JS to TypeScript (static-ts/ → compiled to internal/api/static/)
- Add image resizing on upload: JPEG/PNG/WebP scaled to max 1920px at quality 80
- Extract shared upload logic into upload.go (saveUpload, saveResizedImage, saveResizedWebP)
- Add POST /media endpoint for drag-drop/paste media uploads with markdown ref return
- Add background music player with video/audio coordination (autoplay.ts)
- Add global nav, public feed, hashtags, visibility, Markdown rendering for entries
- Add Dockerfile stage for TypeScript compilation (static-ts-builder)
- Add goldmark, disintegration/imaging, golang.org/x/image dependencies

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/api/webui.go

@@ -13,10 +13,28 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/extension"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/renderer/html"
 	"github.com/jacek/pamietnik/backend/internal/auth"
 	"github.com/jacek/pamietnik/backend/internal/db"
 )
 
+var md = goldmark.New(
+	goldmark.WithExtensions(extension.GFM),
+	goldmark.WithParserOptions(parser.WithAutoHeadingID()),
+	goldmark.WithRendererOptions(html.WithHardWraps()),
+)
+
+func renderMarkdown(src string) template.HTML {
+	var buf bytes.Buffer
+	if err := md.Convert([]byte(src), &buf); err != nil {
+		return template.HTML(template.HTMLEscapeString(src))
+	}
+	return template.HTML(buf.String())
+}
+
 //go:embed static templates
 var assets embed.FS
 
@@ -28,7 +46,11 @@ var funcMap = template.FuncMap{
 		}
 		return *p
 	},
-	"join": strings.Join,
+	"join":     strings.Join,
+	"isVideo":  func(mime string) bool { return strings.HasPrefix(mime, "video/") },
+	"isAudio":  func(mime string) bool { return strings.HasPrefix(mime, "audio/") },
+	"isImage":  func(mime string) bool { return strings.HasPrefix(mime, "image/") },
+	"markdown": renderMarkdown,
 }
 
 var tmpls = template.Must(