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/tidal/client.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"streamrip-go/internal/config"
+	"streamrip-go/internal/jsonutil"
 	"streamrip-go/internal/netutil"
 	"streamrip-go/internal/provider"
 	"streamrip-go/internal/ratelimit"
@@ -149,7 +150,7 @@ func (c *Client) refreshAccessToken(ctx context.Context) error {
 	}
 
 	newRefresh := stringify(resp["refresh_token"])
-	expiresIn := int64(intFromAny(resp["expires_in"]))
+	expiresIn := int64(jsonutil.IntFromAny(resp["expires_in"]))
 	if expiresIn <= 0 {
 		expiresIn = 7 * 24 * 3600
 	}
@@ -773,19 +774,3 @@ func tidalImageMap(cover string) map[string]any {
 		"original":   base + "/1280x1280.jpg",
 	}
 }
-
-func intFromAny(v any) int {
-	switch t := v.(type) {
-	case int:
-		return t
-	case int64:
-		return int(t)
-	case float64:
-		return int(t)
-	case string:
-		i, _ := strconv.Atoi(t)
-		return i
-	default:
-		return 0
-	}
-}