Per-site favicon via site.yaml

A content repo can ship a favicon (svg/png/ico) named in site.yaml;
portal serves it at /site/<path> and injects a <link rel=icon> after
the static defaults so it wins. Also serves .ico assets.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y42TyF8Zu7NGRR2893vNcZ

Cargo.lock

@@ -2948,7 +2948,7 @@ dependencies = [
 
 [[package]]
 name = "portal"
-version = "0.3.30"
+version = "0.3.31"
 dependencies = [
  "anyhow",
  "arc-swap",