feat: CRAWLER_KEEP_BROWSER_OPEN waits for Ctrl+C in headed mode (0.26.0)

Debug aid: when set in headed mode, the crawler blocks on Ctrl+C at
every shutdown point (early auth bails + normal completion) instead
of closing the browser immediately. Operator can inspect DOM, cookies,
and network state in the visible Chromium window before exit.

Ignored in headless (no window to inspect) — logged as a warning if
set under headless so the operator doesn't sit waiting.

chromiumoxide's `Browser` is `kill_on_drop`, so the close-or-wait
helper must await Ctrl+C *before* the Handle is dropped — otherwise
the Chromium child gets killed out from under the operator.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.25.0",
+  "version": "0.26.0",
   "private": true,
   "type": "module",
   "scripts": {