feat(visualization): add scenario export/import with .ccscenario file format

Add ScenarioFile interface and .ccscenario file format for sharing
scenarios between users. Export creates a clean JSON file stripping
runtime fields; import validates schema, deduplicates against existing
scenarios, and persists to IndexedDB. Built-in scenarios now explicitly
set colorMode to weightedGradient.

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