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/upload.go

@@ -0,0 +1,125 @@
+package api
+
+import (
+	"bytes"
+	"image"
+	"image/jpeg"
+	_ "image/jpeg"
+	"image/png"
+	_ "image/png"
+	"io"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/disintegration/imaging"
+	"golang.org/x/image/webp"
+)
+
+const maxImageDimension = 1920
+const jpegQuality = 80
+
+// saveUpload writes an uploaded file to uploadDir, resizing images where applicable.
+// peeked contains the first bytes already read for MIME detection.
+// Returns the saved filename (including extension).
+func saveUpload(uploadDir, baseName, mime string, peeked []byte, rest io.Reader) (string, error) {
+	full := io.MultiReader(bytes.NewReader(peeked), rest)
+	switch mime {
+	case "image/jpeg", "image/png":
+		return saveResizedImage(uploadDir, baseName, mime, full)
+	case "image/webp":
+		return saveResizedWebP(uploadDir, baseName, full)
+	default:
+		ext := allowedMIME[mime]
+		dest := filepath.Join(uploadDir, baseName+ext)
+		return baseName + ext, saveRaw(dest, full)
+	}
+}
+
+func saveResizedImage(uploadDir, baseName, mime string, r io.Reader) (string, error) {
+	img, _, err := image.Decode(r)
+	if err != nil {
+		// Fallback: save raw if decode fails
+		slog.Warn("image decode failed, saving raw", "mime", mime, "err", err)
+		return "", err
+	}
+
+	b := img.Bounds()
+	if b.Dx() > maxImageDimension || b.Dy() > maxImageDimension {
+		img = imaging.Fit(img, maxImageDimension, maxImageDimension, imaging.Lanczos)
+	}
+
+	ext := allowedMIME[mime]
+	filename := baseName + ext
+	dest := filepath.Join(uploadDir, filename)
+	out, err := os.Create(dest)
+	if err != nil {
+		return "", err
+	}
+	defer out.Close()
+
+	switch mime {
+	case "image/jpeg":
+		err = jpeg.Encode(out, img, &jpeg.Options{Quality: jpegQuality})
+	case "image/png":
+		err = png.Encode(out, img)
+	}
+	if err != nil {
+		os.Remove(dest)
+		return "", err
+	}
+	return filename, nil
+}
+
+func saveResizedWebP(uploadDir, baseName string, r io.Reader) (string, error) {
+	img, err := webp.Decode(r)
+	if err != nil {
+		slog.Warn("webp decode failed, saving raw", "err", err)
+		return "", err
+	}
+
+	b := img.Bounds()
+	if b.Dx() > maxImageDimension || b.Dy() > maxImageDimension {
+		img = imaging.Fit(img, maxImageDimension, maxImageDimension, imaging.Lanczos)
+	}
+
+	// Re-encode as JPEG (no pure-Go WebP encoder available)
+	filename := baseName + ".jpg"
+	dest := filepath.Join(uploadDir, filename)
+	out, err := os.Create(dest)
+	if err != nil {
+		return "", err
+	}
+	defer out.Close()
+
+	if err := jpeg.Encode(out, img, &jpeg.Options{Quality: jpegQuality}); err != nil {
+		os.Remove(dest)
+		return "", err
+	}
+	return filename, nil
+}
+
+func saveRaw(dest string, r io.Reader) error {
+	out, err := os.Create(dest)
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+	_, err = io.Copy(out, r)
+	return err
+}
+
+// mimeCategory returns "image", "video", "audio" or "other".
+func mimeCategory(mime string) string {
+	switch {
+	case strings.HasPrefix(mime, "image/"):
+		return "image"
+	case strings.HasPrefix(mime, "video/"):
+		return "video"
+	case strings.HasPrefix(mime, "audio/"):
+		return "audio"
+	default:
+		return "other"
+	}
+}