Polish final demo, errors, and production readiness

[amaruki]

What to build

Harden final demo behavior, recoverable errors, empty states, fallback copy, and production readiness checks.

Acceptance criteria

• Final demo routes, branded recoverable errors, privacy-safe empty states, and share preview polish are complete.
• Secret-only configuration, health endpoint behavior, reset-state production absence, and component fallbacks are verified.
• Manual UI verification covers golden path, loading, empty, and error states.

Blocked by

#29

Slice metadata

• Type: AFK
• Sprint: Sprint 5
• User stories covered: US-14, US-15, US-18, US-19