fix: add trailing slashes to settings and prompts API calls

Without trailing slash, FastAPI's SPAStaticFiles catch-all intercepts
/api/settings and /api/prompts before the API router, returning HTML
instead of JSON (405 error in UI).

Affected endpoints:
- GET /api/settings → /api/settings/
- PUT /api/settings → /api/settings/
- GET /api/prompts → /api/prompts/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/hooks.ts

@@ -37,7 +37,7 @@ import type {
 export function useSettings() {
   return useQuery<Settings>({
     queryKey: ['settings'],
-    queryFn: () => apiGet<Settings>('/settings'),
+    queryFn: () => apiGet<Settings>('/settings/'),
     staleTime: 60_000,
   })
 }
@@ -56,7 +56,7 @@ export function useSettingsStatus() {
 export function useUpdateSettings() {
   const queryClient = useQueryClient()
   return useMutation<Settings, Error, Partial<Settings>>({
-    mutationFn: (settings) => apiPut<Settings>('/settings', settings),
+    mutationFn: (settings) => apiPut<Settings>('/settings/', settings),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['settings'] })
     },
@@ -199,7 +199,7 @@ export function useDownloadEditedCsv() {
 export function usePromptList() {
   return useQuery<PromptListResponse>({
     queryKey: ['prompts'],
-    queryFn: () => apiGet<PromptListResponse>('/prompts'),
+    queryFn: () => apiGet<PromptListResponse>('/prompts/'),
     staleTime: 30_000,
   })
 }