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/provider/qobuz/client.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"streamrip-go/internal/config"
+	"streamrip-go/internal/jsonutil"
 	"streamrip-go/internal/netutil"
 	"streamrip-go/internal/provider"
 	"streamrip-go/internal/ratelimit"
@@ -694,7 +695,7 @@ func (c *Client) fetchAppIDAndSecrets(ctx context.Context) (string, []string, er
 
 	tzNames := make([]string, 0, len(ordered))
 	for _, o := range ordered {
-		tzNames = append(tzNames, strings.Title(o.timezone))
+		tzNames = append(tzNames, jsonutil.TitleCase(o.timezone))
 	}
 	infoRe := regexp.MustCompile(fmt.Sprintf(infoExtrasTemplate, strings.Join(tzNames, "|")))
 	idxInfo := infoRe.SubexpIndex("info")