docs: bete frontend redesign spec — IMPHNEN deep integration
Full 5-layer design document covering:
- Layer 0: Theme tokens & dark mode palette (light + dark CSS vars)
- Layer 1: Shared UI components (Badge, Toast, Skeleton fixes + new primitives)
- Layer 2: Layout & navigation (sidebar expanded, brand bar, tab strip, mobile)
- Layer 3: Feature components (MessageCard, Dashboard, Live panel refinements)
- Layer 4: Polish & micro-interactions (animations, empty states, theme toggle, mascot)
Resolves known issues in DESIGN_TOKENS.md §13 (hardcoded hex colors, z-index,
shimmer mismatch). Adds dark mode support across all components.