refactor: extract session navigation helper

[kilo-WATT]

Summary

• Extracted previous/next session navigation calculations out of SessionDetail.tsx
• Added a pure sessionNavigation helper for current, previous, and next session state
• Added focused tests for the extracted navigation behavior

Behavior

No intended behavior changes.

Preserved:

• Same session list API call
• Same date sorting behavior
• Same /sessions/:date routes
• Same conditional rendering of Previous/Next buttons

Validation

• npm run build from repo root: passed
• npm run build in frontend: passed, including tsc -b
• npm run test in frontend: passed, 3 files / 9 tests
• npm run typecheck: no script exists at root or frontend
• npm run lint in frontend: fails on existing repo-wide lint issues unrelated to this PR
• Docker/LXC build and UI smoke test: passed

Notes

This PR keeps the session detail UI behavior the same while moving navigation calculation into a small helper that is easier to test and maintain.