fix(crawler): relaunch chromium on CDP / nav-timeout errors (0.36.3)

BrowserManager only re-launched chromium when the cached handle was
None. A crash mid-pass left the handle Some pointing at a dead
process — every subsequent acquire returned the zombie Browser, and
every nav cascaded CDP errors until the idle reaper fired.

Add BrowserManager::invalidate(): take the inner mutex, drop the
handle (closing it if present), and signal the next acquire to
relaunch. Idempotent — invalidating an empty handle is a no-op.

Wire detection via NavError::is_likely_browser_dead and a
chain-walking anyhow_looks_browser_dead helper: substring-match
common channel/connection/transport/WebSocket markers and surface
NavError::Timeout as "presumed dead." Apply at both error
boundaries — RealChapterDispatcher::dispatch and
RealMetadataPass::run.

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

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.36.2"
+version = "0.36.3"
 edition = "2021"
 default-run = "mangalord"