feat: add CLI with TUI, self-update, install script, and terminal section on landing page

- Full-screen Bubble Tea TUI with cream background fill using PadLine/FillBlankLines
- Self-update command (--update) pulling from GitHub releases
- install.sh for curl one-liner installation
- Terminal Lovers section on web landing page with install command and CLI features
- All 7 format categories, glob/directory batch support, auto-download ffmpeg

cli/internal/converter/image.go

@@ -0,0 +1,106 @@
+package converter
+
+import (
+	"fmt"
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"os"
+	"strings"
+
+	"golang.org/x/image/bmp"
+	"golang.org/x/image/tiff"
+	"golang.org/x/image/webp"
+)
+
+func convertImage(inputPath, outputPath, targetFormat string) error {
+	f, err := os.Open(inputPath)
+	if err != nil {
+		return fmt.Errorf("opening image: %w", err)
+	}
+	defer f.Close()
+
+	// Decode input — Go's image package auto-registers png, jpeg, gif via import
+	// We also need x/image decoders for bmp, tiff, webp
+	img, format, err := image.Decode(f)
+	if err != nil {
+		// Try specific decoders as fallback
+		f.Seek(0, 0)
+		img, err = tryDecodeImage(f, inputPath)
+		if err != nil {
+			return fmt.Errorf("decoding image (%s): %w", format, err)
+		}
+	}
+
+	out, err := os.Create(outputPath)
+	if err != nil {
+		return fmt.Errorf("creating output: %w", err)
+	}
+	defer out.Close()
+
+	target := strings.ToLower(targetFormat)
+	switch target {
+	case "png":
+		return png.Encode(out, img)
+	case "jpg", "jpeg":
+		return jpeg.Encode(out, img, &jpeg.Options{Quality: 92})
+	case "gif":
+		return gif.Encode(out, img, &gif.Options{NumColors: 256})
+	case "bmp":
+		return bmp.Encode(out, img)
+	case "tiff", "tif":
+		return tiff.Encode(out, img, &tiff.Options{Compression: tiff.Deflate})
+	case "webp":
+		// Go doesn't have a webp encoder in stdlib. Use ffmpeg as fallback.
+		out.Close()
+		os.Remove(outputPath)
+		return convertImageViaFFmpeg(inputPath, outputPath, target)
+	case "avif":
+		out.Close()
+		os.Remove(outputPath)
+		return convertImageViaFFmpeg(inputPath, outputPath, target)
+	case "ico":
+		// ICO is just a small PNG wrapped in ICO container for simple cases.
+		// We'll convert to PNG via ffmpeg or write a 256x256 PNG for now.
+		out.Close()
+		os.Remove(outputPath)
+		return convertImageViaFFmpeg(inputPath, outputPath, target)
+	default:
+		out.Close()
+		os.Remove(outputPath)
+		return fmt.Errorf("unsupported image target format: %s", target)
+	}
+}
+
+func tryDecodeImage(f *os.File, path string) (image.Image, error) {
+	ext := strings.ToLower(path)
+	switch {
+	case strings.HasSuffix(ext, ".webp"):
+		return webp.Decode(f)
+	case strings.HasSuffix(ext, ".bmp"):
+		return bmp.Decode(f)
+	case strings.HasSuffix(ext, ".tiff"), strings.HasSuffix(ext, ".tif"):
+		return tiff.Decode(f)
+	default:
+		return nil, fmt.Errorf("unable to decode image: %s", path)
+	}
+}
+
+func convertImageViaFFmpeg(inputPath, outputPath, format string) error {
+	args := []string{"-y", "-i", inputPath}
+
+	switch format {
+	case "webp":
+		args = append(args, "-quality", "90", outputPath)
+	case "avif":
+		args = append(args, "-c:v", "libaom-av1", "-still-picture", "1", outputPath)
+	case "ico":
+		// Scale to 256x256 for ICO
+		args = append(args, "-vf", "scale=256:256:force_original_aspect_ratio=decrease,pad=256:256:(ow-iw)/2:(oh-ih)/2", outputPath)
+	default:
+		args = append(args, outputPath)
+	}
+
+	return mediaConvert(inputPath, outputPath, format, args)
+}