Refactor: comprehensive cleanup and modularization

- Extracted common JSON parsing helpers into internal/jsonutil
- Removed duplicated helper functions from provider packages
- Removed dead code in internal/app/app.go and downloader.go
- Replaced deprecated strings.Title with jsonutil.TitleCase
- Added graceful shutdown with signal handling in main.go
- Split monolithic cmd/rip/main.go into args.go, helpers.go, lastfm.go, search.go

internal/download/downloader.go

@@ -553,31 +553,6 @@ const deezerBFChunkSize = 2048
 
 var deezerBFIV = []byte{0, 1, 2, 3, 4, 5, 6, 7}
 
-func decryptDeezerBFCBCStripe(in []byte, trackID string) ([]byte, error) {
-	block, err := blowfish.NewCipher(deriveDeezerBlowfishKey(trackID))
-	if err != nil {
-		return nil, err
-	}
-	out := make([]byte, len(in))
-	for i := 0; i*deezerBFChunkSize < len(in); i++ {
-		start := i * deezerBFChunkSize
-		end := start + deezerBFChunkSize
-		if end > len(in) {
-			end = len(in)
-		}
-		chunk := in[start:end]
-		if i%3 == 0 && len(chunk) == deezerBFChunkSize {
-			dec := make([]byte, len(chunk))
-			mode := cipher.NewCBCDecrypter(block, deezerBFIV)
-			mode.CryptBlocks(dec, chunk)
-			copy(out[start:end], dec)
-		} else {
-			copy(out[start:end], chunk)
-		}
-	}
-	return out, nil
-}
-
 func deriveDeezerBlowfishKey(trackID string) []byte {
 	sum := md5.Sum([]byte(trackID))
 	md5Hex := fmt.Sprintf("%x", sum)
@@ -588,20 +563,3 @@ func deriveDeezerBlowfishKey(trackID string) []byte {
 	}
 	return key
 }
-
-func normalizeDeezerTrackID(raw string) string {
-	trimmed := strings.TrimSpace(raw)
-	if trimmed == "" {
-		return ""
-	}
-	if _, err := strconv.Atoi(trimmed); err == nil {
-		return trimmed
-	}
-	parts := strings.Split(strings.Trim(trimmed, "/"), "/")
-	for i := len(parts) - 1; i >= 0; i-- {
-		if _, err := strconv.Atoi(parts[i]); err == nil {
-			return parts[i]
-		}
-	}
-	return trimmed
-}