field-fix: POST/PUT/PATCH without body sends '{}' in mobile api helper

- api() in apps/mobile/src/lib/api.ts now sends JSON.stringify({}) for
  methods with a body slot when no body is provided, instead of sending
  undefined with content-type: application/json.
- Fixes scan start crash (POST /v1/scans/:id/start) and latent
  resend-verification crash (POST /v1/auth/resend-verification).
- Server strictness left intact; uploadImage unchanged.
- Added unit tests for GET/HEAD/POST/PUT body behaviour.

apps/mobile/src/lib/api.ts

@@ -63,13 +63,15 @@ export async function api<T = unknown>(
   options: { method?: string; body?: unknown; retry?: boolean } = {},
 ): Promise<T> {
   const { accessToken } = useAuth.getState();
+  const method = options.method ?? "GET";
+  const hasBodySlot = method !== "GET" && method !== "HEAD";
   const res = await fetch(`${API_BASE}${path}`, {
-    method: options.method ?? "GET",
+    method,
     headers: {
       "content-type": "application/json",
       ...(accessToken ? { authorization: `Bearer ${accessToken}` } : {}),
     },
-    body: options.body != null ? JSON.stringify(options.body) : undefined,
+    body: hasBodySlot ? JSON.stringify(options.body ?? {}) : undefined,
   });
 
   if (res.status === 401 && options.retry !== false) {