feat(slides): add persistent branding with per-slide visibility control

- Add branding overlays: logo, title, author, and footer text
- Use Reveal.js theme CSS variables for automatic color adaptation
- Inline SVGs for currentColor support on any theme
- Per-slide control via HTML comments:
  - <!-- no-branding -->: hide all elements
  - <!-- no-header -->: hide logo + title
  - <!-- no-footer -->: hide author + footer
- Fix body-level CSS selectors for proper visibility
- Add comprehensive documentation in example slides

templates/markdown-slides/assets/media/slides-logo.svg

@@ -0,0 +1 @@
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg"><title>Markdown</title><path fill="currentColor" d="M22.27 19.385H1.73A1.73 1.73 0 010 17.655V6.345a1.73 1.73 0 011.73-1.73h20.54A1.73 1.73 0 0124 6.345v11.308a1.73 1.73 0 01-1.73 1.731zM5.769 15.923v-4.5l2.308 2.885 2.307-2.885v4.5h2.308V8.078h-2.308l-2.307 2.885-2.308-2.885H3.46v7.847zM21.232 12h-2.309V8.077h-2.307V12h-2.308l3.461 4.039z"/></svg>