feat(02-01): prompt editor UI with types, hooks, page, route, sidebar

- Add PromptInfo, PromptListResponse, PromptDetail types
- Add usePromptList, usePrompt, useSavePrompt, useResetPrompt hooks
- Create PromptEditor page with two-column layout, live variables
- Add /prompt-editor route in App.tsx
- Add Prompt Editor nav item with Pencil icon in Sidebar
- TypeScript compiles without errors

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

frontend/src/App.tsx

@@ -5,6 +5,7 @@ import { Dashboard } from './pages/Dashboard'
 import { GenerateCalendar } from './pages/GenerateCalendar'
 import { GenerateSingle } from './pages/GenerateSingle'
 import { OutputReview } from './pages/OutputReview'
+import { PromptEditor } from './pages/PromptEditor'
 import { Settings } from './pages/Settings'
 
 const queryClient = new QueryClient({
@@ -27,6 +28,7 @@ function App() {
             <Route path="/genera" element={<GenerateCalendar />} />
             <Route path="/genera-singolo" element={<GenerateSingle />} />
             <Route path="/risultati/:jobId" element={<OutputReview />} />
+            <Route path="/prompt-editor" element={<PromptEditor />} />
             <Route path="/impostazioni" element={<Settings />} />
           </Routes>
         </Layout>