fix: tag color bug, FormData interceptor, falsy id check

- Fix tag using category_color instead of its own color when editing
- Add FormData auto-detection interceptor in client.ts (root cause of 422)
- Remove redundant Content-Type overrides from all 5 upload endpoints
- Fix falsy id check (id: 0 treated as null) with strict comparison
- Await invalidateQueries to prevent stale dropdown data

src/api/client.ts

@@ -82,6 +82,11 @@ function isAuthEndpoint(url: string | undefined): boolean {
 }
 
 apiClient.interceptors.request.use(async (config: InternalAxiosRequestConfig) => {
+  // Let axios set the correct multipart/form-data header with boundary for FormData
+  if (config.data instanceof FormData && config.headers) {
+    delete config.headers['Content-Type'];
+  }
+
   if (!isAuthEndpoint(config.url)) {
     let token = tokenStorage.getAccessToken();