Frontend: animated skeleton screens replacing spinners

[luaroncrew]

Summary

• Showcase grid: 5 skeleton cards pulse while /api/showcase loads, each matching the real .showcase-card shape (square chart image area + #id pill)
• Inference loading: full result-card skeleton shown while diffusion runs — labels row, 3-panel image strip (real selected chart in slot 1, two grey blocks for the generated/target slots), meta row with badge/confidence/match placeholders, prompt block, reasoning block
• Staggered animation-delay (150ms per card / 200ms per image panel) for a ripple effect
• Pure CSS @keyframes pulse (opacity 1→0.3→1, 1.6s ease-in-out) — zero JS, zero deps

Test plan

• Load page: 5 grey pulsing cards appear before showcase data arrives
• Click a chart card: result skeleton appears immediately with the selected chart in the first image slot
• Verify skeleton collapses cleanly when real result renders
• Check mobile layout (< 700px): skeleton panels stack vertically