Add web upload: drag-and-drop images to depot via browser

- POST /api/upload saves files to /depot/ for ingest processing
- Batches of 10 files per request
- Drag-and-drop zone + file picker, per-file status feedback
- New 'Last opp'-tab in navbar

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

frontend/src/App.jsx

@@ -3,6 +3,7 @@ import StartlistPage from './pages/StartlistPage.jsx'
 import ReviewPage from './pages/ReviewPage.jsx'
 import ResultsPage from './pages/ResultsPage.jsx'
 import PassagesPage from './pages/PassagesPage.jsx'
+import UploadPage from './pages/UploadPage.jsx'
 import './App.css'
 
 function Nav() {
@@ -14,6 +15,7 @@ function Nav() {
       <NavLink to="/passages" className={linkClass}>Passeringer</NavLink>
       <NavLink to="/review" className={linkClass}>Gjennomgang</NavLink>
       <NavLink to="/results" className={linkClass}>Resultater</NavLink>
+      <NavLink to="/upload" className={linkClass}>Last opp</NavLink>
     </nav>
   )
 }
@@ -28,6 +30,7 @@ export default function App() {
           <Route path="/passages" element={<PassagesPage />} />
           <Route path="/review" element={<ReviewPage />} />
           <Route path="/results" element={<ResultsPage />} />
+          <Route path="/upload" element={<UploadPage />} />
         </Routes>
       </main>
     </>