fix: disable preview for custom app

[null-crafter]

Checklist

• Tests were added/updated according to the feature/bugfix/change made
• Version was rolled according to semver requirements
• API endpoints openapi schema was updated if applicable

Changes

Disable preview mode for custom app (having custom-app=true in query param). This enables external app devs to chat with mentors

[claude]

Code Quality Review

• Quality: Clean, minimal change — isCustomApp is well-named and properly integrated into the existing preview-mode condition. Test file changes are purely formatting (single → double quotes).
• Issues: None found — the boolean flag from searchParams.get("custom-app") === "true" is safe and follows existing patterns in the file.
• Suggestions: Looks good. The !isCustomApp && guard is correctly placed as the first condition in the chain, short-circuiting the rest.

Summary

Small, focused fix that disables preview mode for custom-app embeds. Logic is correct and the accompanying test coverage targets the new query param behavior directly.

[claude]

UI/UX Review

• Accessibility: No UI changes introduced; no accessibility impact detected.
• UX Concerns: The custom-app=true flag silently disables preview mode with no user-facing indicator. If the param is misspelled (e.g. custom_app), the external developer silently ends up in preview mode instead — no error or warning.
• Suggestions: Consider a dev-only console warning when custom-app is detected, and document the exact param name clearly for integrators to avoid silent failures.

Summary

Focused, low-risk 3-line fix that correctly bypasses preview mode for custom app integrations. No visual or accessibility regressions. Main UX risk is silent fallback when the query param is mistyped.