feat(update): add rolling dev update channel for per-commit builds

Adds an opt-in Dev channel so panels running CI per-commit builds can self-update to the latest commit, mirroring the stable online-update flow.

CI publishes/overwrites a single fixed-tag pre-release (dev-latest), force-moved to the newest main commit and marked --latest=false so releases/latest stays the stable tag. Builds stamp the short commit via -ldflags; the panel compares the running commit to the dev release commit to detect an update, and update.sh honors XUI_UPDATE_TAG to install from that tag. Linux/systemd only.

frontend/src/pages/api-docs/endpoints.ts

@@ -400,6 +400,12 @@ export const sections: readonly Section[] = [
         path: '/panel/api/server/updatePanel',
         summary: 'Self-update the panel to the latest version. The server restarts on success.',
       },
+      {
+        method: 'POST',
+        path: '/panel/api/server/setUpdateChannel',
+        summary: 'Toggle the panel update channel between stable and the rolling per-commit dev release. Only effective on dev builds.',
+        body: '{\n  "dev": true\n}',
+      },
       {
         method: 'POST',
         path: '/panel/api/server/updateGeofile',