feat(api): package skeleton + error envelope

Introduces internal/api with Mount(), the {error:{code,message}}
response shape, and stubbed login/logout/me so later tasks can
land one handler at a time without breaking the build.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/api/errors.go

@@ -0,0 +1,25 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// errorBody is the JSON envelope for failures on /api/*. Kept separate from
+// the Subsonic envelope so native clients don't have to parse two shapes.
+type errorBody struct {
+	Error errorPayload `json:"error"`
+}
+
+type errorPayload struct {
+	Code    string `json:"code"`
+	Message string `json:"message"`
+}
+
+// writeErr is the canonical way to emit a failure. Code is a stable
+// short-string clients can switch on; message is human-readable.
+func writeErr(w http.ResponseWriter, status int, code, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(errorBody{Error: errorPayload{Code: code, Message: message}})
+}