feat: add admin password change via /settings page

Backend:
- DB: add UpdateAdminPasswordHash(id, hash) in queries_users.go
- Auth: add ChangePassword(username, old, new) method that verifies
  old password, validates length >= 8, and stores bcrypt hash
- Handlers: add handleChangePassword on PUT /api/auth/password (auth required)
- Router: register route inside protected group

Frontend:
- api.ts: add changePassword(oldPassword, newPassword)
- Settings.tsx: new page with form (current + new + confirm), client-side
  validation, success/error feedback
- App.tsx: add /settings route
- Layout.tsx: add 'Ajustes' nav item

web/src/App.tsx

@@ -9,6 +9,7 @@ import Users from "./pages/Users";
 import Samba from "./pages/Samba";
 import Nfs from "./pages/Nfs";
 import Log from "./pages/Log";
+import Settings from "./pages/Settings";
 
 type AuthState = { loading: boolean; authenticated: boolean; username: string };
 
@@ -54,6 +55,7 @@ export default function App() {
           <Route path="/samba" element={<Samba />} />
           <Route path="/nfs" element={<Nfs />} />
           <Route path="/log" element={<Log />} />
+          <Route path="/settings" element={<Settings />} />
           <Route path="*" element={<Navigate to="/" replace />} />
         </Route>
       </Routes>