fix: resolve build error and hydration mismatch from recent cleanup

[mackeh]

• dna.ts: remove dangling sessions++ that referenced a deleted variable (would fail to compile)
• Onboarding.tsx: restore post-mount useEffect for the onboarding check; reading localStorage in a useState initializer caused an SSR hydration mismatch. Extract the storage key into a named constant.
• calendar events route: drop stale comment orphaned by the userId removal

https://claude.ai/code/session_01YFnkjNWLYpJR12r4HLGsvK

Description

Please include a summary of the change and which issue is fixed.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings