docs: add UX polish design spec (4-layer approach)
Defines the UX improvement strategy for Oikos post-v0.1.0:
Layer 1 — Design language & consistency (tokens, typography, module accents)
Layer 2 — Animations & transitions (page transitions, staggering, micro-interactions)
Layer 3 — Mobile PWA & native feel (install prompt, scroll, vibration, keyboard)
Layer 4 — Forms & modals (auto-focus, inline validation, bottom sheets, submit feedback)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Ulas
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