Add dynamic crystal favicon colored per mineral

SVG gem shape as inline data URI, automatically tinted with the
mineral's color_hex. No static assets needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dailystone/templates/dailystone/stone.html

@@ -5,6 +5,9 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{% if mineral %}{{ mineral.name }} — Daily Stone{% else %}Daily Stone{% endif %}</title>
+    {% if mineral %}
+    <link rel="icon" type="image/svg+xml" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 32 32'%3E%3Cpolygon points='16,2 27,12 16,30 5,12' fill='%23{{ mineral.color_hex|cut:'#' }}'/%3E%3Cpolygon points='16,2 27,12 16,14 5,12' fill='white' fill-opacity='0.25'/%3E%3Cpolygon points='27,12 16,30 16,14' fill='black' fill-opacity='0.2'/%3E%3Cpolygon points='16,2 19,12 16,14 13,12' fill='white' fill-opacity='0.15'/%3E%3C/svg%3E">
+    {% endif %}
     <style>
         :root {
             --stone-color: {{ mineral.color_hex|default:"#6b7280" }};