mavweb: ui design system refresh — shared nav, cards, component classes

- Rewrote ui.css with design tokens, card/btn/badge/dot components
- All pages wrapped in <main class=page> with max-width container
- Replaced inline <style> blocks with ui.css classes
- passkey page now uses shared nav.site template
- PWA voice page unified under shared nav.site (no more separate tab nav)
- Inline lang toggle moved from nav to voice page body

cmd/mavweb/trace.html

@@ -3,8 +3,9 @@
 <title>maven · rule trace</title>
 <link rel=stylesheet href=/ui.css>
 {{template "nav" "trace"}}
+<main class=page>
 <h1>rule trace</h1>
-<div class=updated>{{.Now | ago}} — winner: <strong>{{if .Winner}}{{.Winner}}{{else}}nothing fired{{end}}</strong></div>
+<div class=updated style=margin-bottom:var(--space-lg)>{{.Now | ago}} — winner: <strong>{{if .Winner}}{{.Winner}}{{else}}nothing fired{{end}}</strong></div>
 <div class=scroll><table class=mono>
 <tr><th>rule<th>sev<th>predicate<th>gate<th>blocked by<th>detail<th>selected<th>lost to</tr>
 {{range .Rules}}<tr>
@@ -25,3 +26,4 @@
 <td>{{.LostTo}}</td>
 </tr>{{end}}
 </table></div>
+</main>