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/media.go

@@ -0,0 +1,64 @@
+package converter
+
+import (
+	"fmt"
+
+	"github.com/noauf/transmute-cli/internal/ffmpeg"
+)
+
+func convertMedia(inputPath, outputPath, targetFormat string) error {
+	args := buildFFmpegArgs(inputPath, outputPath, targetFormat)
+	return mediaConvert(inputPath, outputPath, targetFormat, args)
+}
+
+func mediaConvert(inputPath, outputPath, targetFormat string, args []string) error {
+	if !ffmpeg.IsAvailable() {
+		return fmt.Errorf("ffmpeg is required for %s conversion — run `transmute --install-ffmpeg` to install it", targetFormat)
+	}
+	return ffmpeg.Run(args...)
+}
+
+func buildFFmpegArgs(inputPath, outputPath, targetFormat string) []string {
+	args := []string{"-y", "-i", inputPath}
+
+	switch targetFormat {
+	// Audio
+	case "mp3":
+		args = append(args, "-codec:a", "libmp3lame", "-q:a", "2", outputPath)
+	case "wav":
+		args = append(args, "-codec:a", "pcm_s16le", outputPath)
+	case "flac":
+		args = append(args, "-codec:a", "flac", outputPath)
+	case "ogg":
+		args = append(args, "-codec:a", "libvorbis", "-q:a", "6", outputPath)
+	case "aac":
+		args = append(args, "-codec:a", "aac", "-b:a", "192k", outputPath)
+	case "m4a":
+		args = append(args, "-codec:a", "aac", "-b:a", "192k", outputPath)
+	case "opus":
+		args = append(args, "-codec:a", "libopus", "-b:a", "128k", outputPath)
+
+	// Video
+	case "mp4":
+		args = append(args, "-codec:v", "libx264", "-preset", "medium", "-crf", "23",
+			"-codec:a", "aac", "-b:a", "192k", outputPath)
+	case "webm":
+		args = append(args, "-codec:v", "libvpx-vp9", "-crf", "30", "-b:v", "0",
+			"-codec:a", "libvorbis", outputPath)
+	case "avi":
+		args = append(args, "-codec:v", "mpeg4", "-q:v", "5",
+			"-codec:a", "libmp3lame", "-q:a", "4", outputPath)
+	case "mov":
+		args = append(args, "-codec:v", "libx264", "-preset", "medium", "-crf", "23",
+			"-codec:a", "aac", "-b:a", "192k", outputPath)
+	case "mkv":
+		args = append(args, "-codec:v", "libx264", "-preset", "medium", "-crf", "23",
+			"-codec:a", "aac", "-b:a", "192k", outputPath)
+
+	default:
+		// Generic: let ffmpeg figure it out from the extension
+		args = append(args, outputPath)
+	}
+
+	return args
+}