Slides fixes

[shomix]

A handful of small fixes and polish to the slides editor.

• Mermaid diagrams render correctly. Extracting mermaid blocks before sanitization preserves the -->
  arrows (the DOMParser round-trip in the sanitizer was HTML-escaping > to >, which broke the parser).
  Also widened DOMPurify's SVG allow-list to keep label text (foreignObject, text, tspan, font + text-anchor
  attrs).
• Excalidraw works again. The Excalidraw bundle's CSS is now eagerly imported alongside the lazy
  component, so the canvas renders instead of appearing blank.
• Layout switch seeds content. Picking a layout in the toolbar now also drops in that layout's default
  HTML, instead of leaving the existing content stranded under the new layout name.
• Empty blank slide is no longer empty. Inserting a blank slide now shows a faint "Double-click to edit"
  hint so users have something to click on.
• "Add empty slide" entry in the sidebar's Add Slides popover (next to "Duplicate current slide"), for a
  no-AI starting point.
• Deck rename from the card menu is more reliable — replaced the setTimeout hack with onCloseAutoFocus on
  the dropdown so the rename input focuses every time.
• Typo fix: "Edit IconHistory" → "Edit History" in the history panel.

https://clips.agent-native.com/share/Q5ab3qBNkUGa

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[steve8708]

awesome @shomix !!