fix: add visualization quality guidance to agent prompt

[GeneralJerel]

Summary

• Add "Visualization Quality Standards" section to the agent system prompt
• Documents available import map libraries (Three.js, GSAP, D3, Chart.js) with correct import syntax
• Mandates Three.js for 3D content — explicitly prohibits CSS 3D faking and Canvas 2D projection
• Sets quality bar: polished, portfolio-ready output with proper lighting, antialiasing, responsive sizing
• Requires <script type="module"> when using import map libraries

Context

Generated visualizations have degraded in quality (see #58). The model was producing broken 3D content using CSS transform-style: preserve-3d hacks or Canvas 2D manual projection instead of Three.js, which is available via the import map. The system prompt had no guidance about available libraries or quality expectations.

Test plan

• Deploy updated agent and test "sphere icosahedron morph" prompt — should use Three.js WebGL
• Test other 3D prompts (solar system, neural network) — verify Three.js usage
• Test non-3D prompts (diagrams, charts) — verify no regression
• Verify <script type="module"> is used in generated HTML

Closes #58