[SC-02] Expo Router layout with tabs (voice, chat, settings) + auth gate (retry 1)

## SC-02: Expo Router layout with tabs (voice, chat, settings) + auth gate Section: Scaffold | Milestone: M1 | Owner: AG Lu ### Failing Tests - **root_layout** (file_exists): File `c8-audio/app/_layout.tsx` must exist - **register_globals_root** (grep): Pattern `registerGlobals` in `c8-audio/app/_layout.tsx` (need 1+ matches) - **tabs_layout** (file_exists): File `c8-audio/app/(tabs)/_layout.tsx` must exist - **voice_screen** (file_exists): File `c8-audio/app/(tabs)/voice.tsx` must exist - **chat_screen** (file_exists): File `c8-audio/app/(tabs)/chat.tsx` must exist - **settings_screen** (file_exists): File `c8-audio/app/(tabs)/settings.tsx` must exist - **index_entry** (file_exists): File `c8-audio/app/index.tsx` must exist ### Acceptance Criteria - File `c8-audio/app/_layout.tsx` must exist - File matching `c8-audio/app/_layout.tsx` must contain pattern `registerGlobals` - File `c8-audio/app/(tabs)/_layout.tsx` must exist - File `c8-audio/app/(tabs)/voice.tsx` must exist - File `c8-audio/app/(tabs)/chat.tsx` must exist - File `c8-audio/app/(tabs)/settings.tsx` must exist - File `c8-audio/app/index.tsx` must exist ### Codebase Conventions - Trilingual comments: // EN: ... // VI: ... // ES: ... - SPEC section refs in comments - Zod at module level with .safeParse() - (supabase as any) with eslint-disable - NextResponse.json() responses - Error format: { error: 'message' } ### Dependencies - SC-01: FAIL