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/spreadsheet.go

@@ -0,0 +1,161 @@
+package converter
+
+import (
+	"encoding/csv"
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/xuri/excelize/v2"
+	"gopkg.in/yaml.v3"
+)
+
+func convertSpreadsheet(inputPath, outputPath, sourceExt, targetFormat string) error {
+	// Read spreadsheet using excelize (supports xlsx, xls via xlsx conversion)
+	f, err := excelize.OpenFile(inputPath)
+	if err != nil {
+		return fmt.Errorf("opening spreadsheet: %w", err)
+	}
+	defer f.Close()
+
+	// Get first sheet
+	sheetName := f.GetSheetName(0)
+	if sheetName == "" {
+		return fmt.Errorf("no sheets found in spreadsheet")
+	}
+
+	rows, err := f.GetRows(sheetName)
+	if err != nil {
+		return fmt.Errorf("reading rows: %w", err)
+	}
+
+	if len(rows) == 0 {
+		return fmt.Errorf("spreadsheet is empty")
+	}
+
+	headers := rows[0]
+	dataRows := rows[1:]
+
+	switch targetFormat {
+	case "csv":
+		return writeSpreadsheetDelimited(headers, dataRows, outputPath, ',')
+	case "tsv":
+		return writeSpreadsheetDelimited(headers, dataRows, outputPath, '\t')
+	case "json":
+		return writeSpreadsheetJSON(headers, dataRows, outputPath)
+	case "yaml", "yml":
+		return writeSpreadsheetYAML(headers, dataRows, outputPath)
+	case "xml":
+		return writeSpreadsheetXML(headers, dataRows, outputPath)
+	case "html":
+		return writeSpreadsheetHTML(headers, dataRows, outputPath)
+	default:
+		return fmt.Errorf("unsupported target for spreadsheet: %s", targetFormat)
+	}
+}
+
+func writeSpreadsheetDelimited(headers []string, rows [][]string, outputPath string, sep rune) error {
+	f, err := os.Create(outputPath)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	w := csv.NewWriter(f)
+	w.Comma = sep
+	if err := w.Write(headers); err != nil {
+		return err
+	}
+	for _, row := range rows {
+		// Pad row to match header length
+		for len(row) < len(headers) {
+			row = append(row, "")
+		}
+		if err := w.Write(row[:len(headers)]); err != nil {
+			return err
+		}
+	}
+	w.Flush()
+	return w.Error()
+}
+
+func writeSpreadsheetJSON(headers []string, rows [][]string, outputPath string) error {
+	var records []map[string]string
+	for _, row := range rows {
+		record := make(map[string]string)
+		for i, h := range headers {
+			if i < len(row) {
+				record[h] = row[i]
+			} else {
+				record[h] = ""
+			}
+		}
+		records = append(records, record)
+	}
+	b, err := json.MarshalIndent(records, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(outputPath, b, 0o644)
+}
+
+func writeSpreadsheetYAML(headers []string, rows [][]string, outputPath string) error {
+	var records []map[string]string
+	for _, row := range rows {
+		record := make(map[string]string)
+		for i, h := range headers {
+			if i < len(row) {
+				record[h] = row[i]
+			} else {
+				record[h] = ""
+			}
+		}
+		records = append(records, record)
+	}
+	b, err := yaml.Marshal(records)
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(outputPath, b, 0o644)
+}
+
+func writeSpreadsheetXML(headers []string, rows [][]string, outputPath string) error {
+	var sb strings.Builder
+	sb.WriteString("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<spreadsheet>\n")
+	for _, row := range rows {
+		sb.WriteString("  <row>\n")
+		for i, h := range headers {
+			val := ""
+			if i < len(row) {
+				val = row[i]
+			}
+			sb.WriteString(fmt.Sprintf("    <%s>%s</%s>\n", h, val, h))
+		}
+		sb.WriteString("  </row>\n")
+	}
+	sb.WriteString("</spreadsheet>")
+	return os.WriteFile(outputPath, []byte(sb.String()), 0o644)
+}
+
+func writeSpreadsheetHTML(headers []string, rows [][]string, outputPath string) error {
+	var sb strings.Builder
+	sb.WriteString("<html><body>\n<table border=\"1\">\n<thead>\n<tr>")
+	for _, h := range headers {
+		sb.WriteString("<th>" + h + "</th>")
+	}
+	sb.WriteString("</tr>\n</thead>\n<tbody>\n")
+	for _, row := range rows {
+		sb.WriteString("<tr>")
+		for i := range headers {
+			val := ""
+			if i < len(row) {
+				val = row[i]
+			}
+			sb.WriteString("<td>" + val + "</td>")
+		}
+		sb.WriteString("</tr>\n")
+	}
+	sb.WriteString("</tbody>\n</table>\n</body></html>")
+	return os.WriteFile(outputPath, []byte(sb.String()), 0o644)
+}