Move preview button to per-slot in edit form, remove card dropdown

- Preview button always visible next to Variables for each template slot
- Remove \n\n prefix from video_warning default value
- Use conditional {% if video_warning %} with blank line in templates
- Fix all .jinja2 files and inline defaults to match
- Add SVG favicon (camera + notification dot)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/static/favicon.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="6" fill="#4f46e5"/>
+  <circle cx="16" cy="15" r="7" fill="none" stroke="white" stroke-width="2"/>
+  <circle cx="16" cy="15" r="3" fill="white"/>
+  <rect x="11" y="6" width="10" height="3" rx="1" fill="white" opacity="0.7"/>
+  <circle cx="25" cy="8" r="5" fill="#ef4444"/>
+  <circle cx="25" cy="8" r="3" fill="#ef4444" stroke="white" stroke-width="1.5"/>
+</svg>