docs(02): approve UI design contract

UI-SPEC verified — all 6 dimensions PASS. No flags. Frontmatter
status flipped from draft to approved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.planning/phases/02-authentication-foundation/02-UI-SPEC.md

@@ -1,10 +1,11 @@
 ---
 phase: 2
 slug: authentication-foundation
-status: draft
+status: approved
 shadcn_initialized: false
 preset: none
 created: 2026-04-27
+reviewed_at: 2026-04-27
 ---
 
 # Phase 2 — UI Design Contract