fix(capacity): Fix availability load/save error handling

- Clone response before reading to prevent body consumption issues
- Add better error logging in API client
- Surface backend error messages in capacity page
- Import ApiError type for proper error handling

Test Results:
- Backend: 15 capacity tests passed ✅
- Frontend Unit: 10 passed ✅
- E2E: 130 passed, 24 skipped ✅

Refs: openspec/changes/headroom-foundation

frontend/src/lib/services/api.ts

@@ -221,13 +221,21 @@ export async function apiRequest<T>(endpoint: string, options: ApiRequestOptions
 
 // Handle API response
 async function handleResponse(response: Response): Promise<Response> {
-  const contentType = response.headers?.get?.('content-type') || response.headers?.get?.('Content-Type');
+  const contentType =
+    response.headers?.get?.('content-type') || response.headers?.get?.('Content-Type');
   const isJson = contentType && contentType.includes('application/json');
 
   if (!response.ok) {
-    const data = isJson ? await response.json() : await response.text();
+    const payloadResponse = response.clone();
+    const data = isJson ? await payloadResponse.json() : await payloadResponse.text();
     const errorData = typeof data === 'object' ? data : { message: data };
     const message = (errorData as { message?: string }).message || 'API request failed';
+    console.error('API error', {
+      url: response.url,
+      status: response.status,
+      message,
+      data: errorData
+    });
     throw new ApiError(message, response.status, errorData);
   }