Add animated word cloud tool with Archimedean spiral visualization (#241)

* Add animated word cloud tool with Archimedean spiral visualization

Single-page HTML tool that accepts pasted text and builds a word cloud
using the algorithm from the Rust wordcloud walkthrough, animated to
make each step visible. Features include:
- Text persisted in URL #fragment (base64-encoded) for sharing
- Auto-submit when loaded with populated fragment
- Play/pause animation with adjustable speed slider
- Frame-by-frame stepping when paused
- Progress scrubber to jump to any point in the animation
- PNG download of the current canvas state at any time
- Visual indicators: ghost words for collision probes, bounding boxes,
  crosshairs, and status labels for placed/skipped words
- Grid-based spatial indexing for collision detection
- Logarithmic font scaling for balanced word sizes
- Stop word filtering matching the Rust implementation

https://claude.ai/code/session_01MSBSKkyGbhzjfybwnjRemt

* Hide overlays on final frame and in PNG downloads, bump textarea font to 16px

- Overlays (placed/skipped/trying labels, highlight boxes, stats counter)
  are suppressed on the last animation frame for a clean result
- PNG download always renders a clean frame without overlays, then
  restores the normal view if mid-animation
- Textarea font size increased from 0.9rem to 16px

https://claude.ai/code/session_01MSBSKkyGbhzjfybwnjRemt

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Co-authored-by: Claude <noreply@anthropic.com>

Author: simonw