Replace emoji UI icons with a shared SVG icon system.

Add an icons sprite and helpers so nav, actions, and protocol cards use consistent stroke icons.

Co-authored-by: Cursor <cursoragent@cursor.com>

templates/guest.html

@@ -1,4 +1,5 @@
 {% extends "base.html" %}
+{% from "macros/icons.html" import icon %}
 
 {% block title_extra %} — {{ _('guest_title') }}{% endblock %}
 
@@ -12,7 +13,7 @@
 
     {% if need_password %}
     <div style="padding: var(--space-lg); text-align: center;">
-        <div class="logo-icon" style="font-size: 3rem; margin-bottom: var(--space-md);">🔐</div>
+        <div class="logo-icon" style="width:64px;height:64px;margin:0 auto var(--space-md);">{{ icon('lock') }}</div>
         <p style="margin-bottom: var(--space-md);">{{ _('guest_protected_desc') }}</p>
 
         <form id="authForm" onsubmit="authGuest(event)"