fix(app): supply fallback session titles

[Brendonovich]

Summary

• port the timestamped session-title fallback into the app compatibility utility
• apply fallback titles while normalizing current root and child sessions
• cover untitled session normalization with focused tests

Validation

• bun test --conditions=solid --only-failures --preload ./happydom.ts ./src/utils/session.test.ts
• bun typecheck
• git diff --check