fix: visualization and dashboard scalability for large datasets

[Jovonni]

Summary

Fixes critical performance and rendering issues when working with large numbers of visualizations (2000+).

• Lightweight list endpoint: API list query now excludes heavy rendered_output and code blobs, returning a
  VisualizationSummary instead — response dropped from ~100MB to ~900KB
• Server-side pagination: Added page and per_page query params with LIMIT/OFFSET to the visualizations
  list endpoint (default 24 per page)
• Frontend pagination controls: Visualizations list page now shows paginated results with page navigation
  instead of rendering 2000+ animated cards simultaneously (which caused a 2+ minute browser freeze from stagger
  animations)
• WebGL context fix: Dashboard panels now lazy-render via IntersectionObserver — only visible panels mount
  their VizRenderer, preventing browser WebGL context exhaustion (gl-shader: Error compiling shader: null) when
  dashboards have many 3D Plotly charts
• Placeholder chart fix: Fixed bug where notebook-generated visualizations showed a template Plotly chart
  instead of their actual rendered output on the detail page
• Dashboard compat: Updated dashboard viz picker to handle the new paginated API response format

Test plan

• Open /visualizations with 2000+ visualizations — page loads instantly with pagination controls
• Navigate between pages — cards load quickly with stagger animation
• Open /dashboards/:id with many 3D Plotly panels — no WebGL shader errors, panels lazy-load on scroll
• Open a notebook-created visualization — shows actual rendered output, not placeholder template
• Dashboard "Add Panel" picker still lists available visualizations correctly