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/media.go

@@ -0,0 +1,76 @@
+package api
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"strings"
+)
+
+type MediaHandler struct {
+	uploadDir string
+}
+
+func NewMediaHandler(uploadDir string) *MediaHandler {
+	return &MediaHandler{uploadDir: uploadDir}
+}
+
+// HandleUpload handles POST /media — uploads a single file and returns its markdown reference.
+func (h *MediaHandler) HandleUpload(w http.ResponseWriter, r *http.Request) {
+	if err := r.ParseMultipartForm(maxUploadSize); err != nil {
+		http.Error(w, "too large", http.StatusRequestEntityTooLarge)
+		return
+	}
+	fh, _, err := r.FormFile("file")
+	if err != nil {
+		http.Error(w, "missing file", http.StatusBadRequest)
+		return
+	}
+	defer fh.Close()
+
+	buf := make([]byte, 512)
+	n, _ := fh.Read(buf)
+	mime := http.DetectContentType(buf[:n])
+	if _, ok := allowedMIME[mime]; !ok {
+		http.Error(w, "unsupported type", http.StatusUnsupportedMediaType)
+		return
+	}
+
+	filename, err := saveUpload(h.uploadDir, randomID(), mime, buf[:n], fh)
+	if err != nil {
+		http.Error(w, "storage error", http.StatusInternalServerError)
+		return
+	}
+
+	ref := markdownRef(mime, filename)
+	slog.Info("media uploaded", "filename", filename, "mime", mime)
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(map[string]string{
+		"filename": filename,
+		"mime":     mime,
+		"ref":      ref,
+	})
+}
+
+func markdownRef(mime, filename string) string {
+	url := "/uploads/" + filename
+	switch {
+	case strings.HasPrefix(mime, "image/"):
+		return "![" + filename + "](" + url + ")"
+	case strings.HasPrefix(mime, "video/"):
+		return "![" + filename + "](" + url + ")"
+	case strings.HasPrefix(mime, "audio/"):
+		return "[" + filename + "](" + url + ")"
+	default:
+		return "[" + filename + "](" + url + ")"
+	}
+}
+
+func randomID() string {
+	b := make([]byte, 12)
+	_, _ = rand.Read(b)
+	return hex.EncodeToString(b)
+}