Add configurable default project fallback for blank timesheet assignments

[jaromil]

Summary

• add agiladmin.default-project config normalization and startup validation
• bill positive hours from blank assignment project cells to the configured default project
• thread config-aware parsing through reports, recent project discovery, and upload diff flows
• add browser coverage for the default-project fallback and document the setting

Testing

• clj -M:test
• npm run test:e2e