fix: don't burn a job attempt when the browser is unavailable

A job's attempt is incremented at lease time, then the dispatcher acquires
the headless browser. When the browser was down or mid-restart, acquire()
failed and the error was ack_failed — so during an outage the whole pending
backlog was chewed to `dead` within minutes while workers hot-looped.

Add SyncOutcome::BrowserUnavailable: the dispatcher returns it instead of
propagating the acquire error, and the daemon releases the lease (refunding
the attempt, like the cancel/session paths) and backs off, so an outage is
treated as an infrastructure blip rather than per-job failures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.16",
+  "version": "0.124.17",
   "private": true,
   "type": "module",
   "scripts": {