refactor(auth): replace untyped JWT claims with typed Claims struct

Introduced a typed Claims struct in core/auth to replace the raw
map[string]any approach used for JWT claims throughout the codebase.
This provides compile-time safety and better readability when creating,
validating, and extracting JWT tokens. Also upgraded lestrrat-go/jwx
from v2 to v3 and go-chi/jwtauth to v5.4.0, adapting all callers to
the new API where token accessor methods now return tuples instead of
bare values. Updated all affected handlers, middleware, and tests.

Signed-off-by: Deluan <deluan@navidrome.org>

server/subsonic/middlewares.go

@@ -159,7 +159,7 @@ func validateCredentials(user *model.User, pass, token, salt, jwt string) error
 	switch {
 	case jwt != "":
 		claims, err := auth.Validate(jwt)
-		valid = err == nil && claims["sub"] == user.UserName
+		valid = err == nil && claims.Subject == user.UserName
 	case pass != "":
 		if strings.HasPrefix(pass, "enc:") {
 			if dec, err := hex.DecodeString(pass[4:]); err == nil {