Remove frontend source from cmd

The frontend is built separately and copied into the Docker image from
the musicfrontend image.

.air.toml

@@ -6,7 +6,7 @@ tmp_dir = "tmp"
   bin = "./tmp/main"
   cmd = "go build -o ./tmp/main ./cmd"
   delay = 1000
-  exclude_dir = ["assets", "tmp", "vendor", "testdata", "frontend", "swagger", "cmd/frontend", "cmd/swagger"]
+  exclude_dir = ["assets", "tmp", "vendor", "testdata", "swagger", "cmd/swagger"]
   exclude_file = []
   exclude_regex = ["_test.go"]
   exclude_unchanged = false