fix: apply the BrandingName setting to the masthead and page title

The brandingName admin setting was saved but never used — layout/login/register
hardcoded "Bounty Board". render() now passes the configured brand into every
page, and the top-nav masthead, <title>, and login/register headers use it.
(The "Bounty Board" board-feature labels are left as-is.)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/templates/login.html

@@ -1,7 +1,7 @@
 {{define "content"}}
 <header class="masthead">
   <p class="masthead-kicker">Consulting Work Exchange</p>
-  <p class="masthead-title">◈ Bounty Board</p>
+  <p class="masthead-title">◈ {{.Brand}}</p>
   <p class="masthead-rule">❦</p>
 </header>
 <div class="card">