feat: default crawler browser to headless (0.32.0)

LaunchOptions::from_env() and LaunchOptions::default() now return
BrowserMode::Headless. The in-process daemon (via CrawlerConfig::from_env)
and the standalone crawler binary both pick this up — no display
required for production runs, smaller resource footprint.

`Headed` stays as an explicit opt-in via CRAWLER_BROWSER_MODE=headed
for debugging or sites that fingerprint headless Chrome. New unit test
locks the default in place.

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

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.31.0"
+version = "0.32.0"
 edition = "2021"
 default-run = "mangalord"