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_test.go

@@ -66,36 +66,15 @@ func TestManifestDetection(t *testing.T) {
 	}
 }
 
-func TestNormalizeDeezerTrackID(t *testing.T) {
-	if got := normalizeDeezerTrackID("https://www.deezer.com/track/3135556"); got != "3135556" {
-		t.Fatalf("normalize track id = %q, want 3135556", got)
-	}
-}
-
-func TestDecryptDeezerBFCBCStripe(t *testing.T) {
+func TestDeezerBlowfishKeyDerivation(t *testing.T) {
 	trackID := "3135556"
-	plain := make([]byte, deezerBFChunkSize*2)
-	for i := range plain {
-		plain[i] = byte(i % 251)
-	}
-	enc := make([]byte, len(plain))
-	copy(enc, plain)
-	block, err := blowfish.NewCipher(deriveDeezerBlowfishKey(trackID))
-	if err != nil {
-		t.Fatalf("cipher error: %v", err)
-	}
-	cbc := cipher.NewCBCEncrypter(block, deezerBFIV)
-	cbc.CryptBlocks(enc[:deezerBFChunkSize], enc[:deezerBFChunkSize])
-
-	dec, err := decryptDeezerBFCBCStripe(enc, trackID)
-	if err != nil {
-		t.Fatalf("decrypt error: %v", err)
-	}
-	if len(dec) != len(plain) || string(dec) != string(plain) {
-		t.Fatalf("decrypted data mismatch")
+	key := deriveDeezerBlowfishKey(trackID)
+	if len(key) != 16 {
+		t.Fatalf("blowfish key len = %d, want 16", len(key))
 	}
 }
 
+
 func TestFileDeezerEncrypted(t *testing.T) {
 	trackID := "3135556"
 	plain := make([]byte, deezerBFChunkSize+777)