fix(app): update page title per sub-app

[dttxorg]

Summary

• add a page title service that derives the Runbox sub-app name from the current route
• wire the service into app startup so the document title updates after navigation
• add unit coverage for mail/default routes, sub-app routes, and NavigationEnd updates

Fixes #284

Testing

• npm run lint -- --lint-file-patterns src/app/page-title.service.ts --lint-file-patterns src/app/page-title.service.spec.ts --lint-file-patterns src/app/app.module.ts
• npm run build
• npm test -- --watch=false --include src/app/page-title.service.spec.ts (bundle generation succeeds, but Karma cannot launch because Firefox is not installed in this local environment: "No binary for Firefox browser on your platform. Please, set FIREFOX_BIN env variable.")

AI disclosure

This contribution was implemented with assistance from OpenAI Codex in src/app/page-title.service.ts, src/app/page-title.service.spec.ts, and src/app/app.module.ts.