feat(frontend): redesign UI theme

[HHassancoding]

Problem

The current frontend lacks a consistent, intentional design language. It does not reflect the product's identity or target audience.

Goal

Establish a cohesive visual theme across the full frontend — typography, colour, spacing, motion, and copy tone — calibrated to the aesthetic references defined in frontend.agent.md.

Acceptance Criteria

• Monochrome base palette applied globally (no gradients, no glass morphism, no drop shadows)
• Typography is the primary visual element — sizing, weight, and spacing carry hierarchy
• All colour usage is intentional and minimal — one accent maximum
• Motion is purposeful: reveal, confirm, or transition only — no decorative animations
• Component copy is short, direct, and opinionated — not placeholder or generic
• UI renders correctly on mobile (primary target) and desktop
• Existing phase logic (empty | loading | success) and state contracts are preserved
• All existing tests pass after changes

Scope

• frontend/src/components/ — all components
• frontend/src/ — global styles or CSS variables if introduced
• No changes to frontend/src/api/, frontend/src/constraints/options.js, or backend

References

• Design rules: .github/copilot/agents/frontend.agent.md
• Aesthetic references: nothing.tech, tastelabs.com

Notes

This is a style-only change. Zero logic or API contract modifications.