draft: preview mode follow ups

[royendo]

1. AppMode enum (replacing string literals)
2. Centralized route utility functions with tests (all 8 tests passing, full test suite green)

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!