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/converter.go

@@ -0,0 +1,76 @@
+package converter
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"github.com/noauf/transmute-cli/internal/detect"
+)
+
+// Result holds the outcome of a single conversion.
+type Result struct {
+	InputPath  string
+	OutputPath string
+	Err        error
+}
+
+// Convert is the main entry point. It routes to the correct converter based on
+// the source file's category.
+func Convert(inputPath, targetFormat, outputDir string) Result {
+	ext := strings.TrimPrefix(filepath.Ext(inputPath), ".")
+	ext = strings.ToLower(ext)
+
+	cat := detect.DetectCategory(ext)
+
+	// Determine output path
+	base := strings.TrimSuffix(filepath.Base(inputPath), filepath.Ext(inputPath))
+	dir := filepath.Dir(inputPath)
+	if outputDir != "" {
+		dir = outputDir
+	}
+	outPath := filepath.Join(dir, base+"."+targetFormat)
+
+	// Avoid overwriting — append _converted if output == input
+	if outPath == inputPath {
+		outPath = filepath.Join(dir, base+"_converted."+targetFormat)
+	}
+
+	var err error
+	switch cat {
+	case detect.CategoryImage:
+		err = convertImage(inputPath, outPath, targetFormat)
+	case detect.CategoryDocument:
+		err = convertDocument(inputPath, outPath, ext, targetFormat)
+	case detect.CategoryAudio, detect.CategoryVideo:
+		err = convertMedia(inputPath, outPath, targetFormat)
+	case detect.CategoryData:
+		err = convertData(inputPath, outPath, ext, targetFormat)
+	case detect.CategorySpreadsheet:
+		err = convertSpreadsheet(inputPath, outPath, ext, targetFormat)
+	case detect.CategoryFont:
+		err = convertFont(inputPath, outPath, ext, targetFormat)
+	default:
+		err = fmt.Errorf("unsupported file type: %s", ext)
+	}
+
+	return Result{
+		InputPath:  inputPath,
+		OutputPath: outPath,
+		Err:        err,
+	}
+}
+
+// OutputPath computes what the output path would be without performing conversion.
+func OutputPath(inputPath, targetFormat, outputDir string) string {
+	base := strings.TrimSuffix(filepath.Base(inputPath), filepath.Ext(inputPath))
+	dir := filepath.Dir(inputPath)
+	if outputDir != "" {
+		dir = outputDir
+	}
+	outPath := filepath.Join(dir, base+"."+targetFormat)
+	if outPath == inputPath {
+		outPath = filepath.Join(dir, base+"_converted."+targetFormat)
+	}
+	return outPath
+}