feat(ux): lock UI to welcome agent until chat onboarding completes #883

Description

After the onboarding overlay (UI wizard) completes, the user should be locked into a focused, distraction-free chat with the welcome agent. Currently, the full app UI (bottom tab bar, thread sidebar, thread list) is visible immediately after the overlay — letting users navigate away from the welcome agent conversation before it finishes.

Current behavior

• Onboarding overlay completes → navigates to /chat
• Full UI is immediately visible: bottom tab bar, thread sidebar, all navigation
• User can navigate away from the welcome agent conversation at any time
• chat_onboarding_completed flag exists in Rust core but is not surfaced to the frontend

Expected behavior

1. Onboarding overlay completes → user lands on a locked-down Conversations screen
2. Hidden: bottom tab bar, thread sidebar/list, side navigation — only the single welcome agent conversation thread is visible
3. User is fully locked — no way to escape until the welcome agent calls complete_onboarding with action: "complete"
4. Once chat_onboarding_completed flips to true:
   • Full UI unlocks (bottom nav, sidebar reappear)
   • A new thread opens automatically

Implementation notes

• chat_onboarding_completed needs to be added to CoreAppSnapshot and surfaced through CoreStateProvider (similar to how onboardingCompleted works today)
• BottomTabBar should hide when chat_onboarding_completed is false
• Conversations.tsx thread sidebar should hide when chat_onboarding_completed is false
• The core already has the complete_onboarding tool (src/openhuman/tools/impl/agent/complete_onboarding.rs) with engagement guards (≥3 exchanges or ≥1 Composio integration)
• The frontend needs to poll or subscribe to the flag change so it can react when the welcome agent finalizes