feat: refine Calcifer firefield hero

[ytvee]

Summary

Refines the existing hero into a borderless, full-section firefield based on the supplied visual reference.

• rebuilds the demon as a broader smiling flame silhouette with a tall central crown, raised side arms, large luminous eye rings, dark pupils and a wide open mouth;
• increases graph density and adds clustered starburst sparks plus drifting ember pairs around the silhouette;
• removes the visual frame so the graph occupies the hero canvas directly;
• stretches the interactive canvas across the entire section, while keeping the initial demon composition on the right at desktop widths;
• keeps every node draggable and propagates movement through the spring-connected graph for large full-section deformation;
• adds stronger role-aware idle convection, flicker, edge pulses and particle movement so the demon visibly burns while the user is inactive;
• replaces the single code block with six dense, small code columns covering the whole hero background;
• keeps the code behind the demon and reveals it only through a speed-sensitive radial torch while dragging;
• preserves static HTML copy, adaptive quality, lazy Three.js loading, reduced-motion behavior and the deterministic SVG fallback.

Scope

Only the existing hero and its graph implementation are changed. No additional sections or routes are added.

Verification

GitHub Actions completed successfully:

• dependency installation — passed;
• graph and physics tests — passed;
• Astro strict type checking — passed;
• production build — passed.

[ytvee]