Say env var changes take effect at the next deploy or rollback (closes #255)

The hint under the app page's environment variable editor asked for a
container restart. Environment variables are set when a container is
created, and upaas's restart stops and starts the same container, so it
keeps the old values; deploy and rollback each create a new container.
The hint now says so in one sentence, styled with alert-warning like the
page's Volume Mounts note: its old text-amber-600 class is not in the
built stylesheet. A handler test renders the app page and checks the
hint.

Model: opus-5-5

templates/app_detail.html

@@ -178,7 +178,7 @@
                                     <button type="submit" class="btn-primary text-sm">Save</button>
                                     <button type="button" @click="editIdx = -1" class="text-gray-500 hover:text-gray-700 text-sm">Cancel</button>
                                 </form>
-                                <p class="text-xs text-amber-600 mt-1">⚠ Container restart needed after env var changes.</p>
+                                <p class="alert-warning mt-1">Environment variable changes take effect at the next deploy or rollback.</p>
                             </td>
                         </template>
                     </tr>