feat(desktop): improve macOS chrome and expand preset employees

[coder-zkl1988]

Summary

• allow a unique tenant-scoped display name to be used as a login identifier while preserving username priority
• improve the macOS titlebarless window with a safe drag region and double-click zoom support
• move sidebar branding and chat header controls below the native macOS drag area
• add five preset employees for procurement, project management, sales, data analysis, and marketing
• preserve existing knowledge-base version references during SQLite startup migration
• retain the existing StaffDeck landing/login interface

Why

Regular users need to sign in with the display name shown by StaffDeck after an administrator changes their password. The macOS desktop shell also needs a usable drag target without overlapping page controls. The additional preset employees make the default workspace more complete, while the migration guard prevents legacy knowledge-base rows from conflicting with generated version identifiers.

The proposed replacement login design was removed so this PR keeps the current landing-page experience.

User impact

• users can sign in with either their username or a unique display name within the tenant
• macOS users can drag and double-click zoom the titlebarless window without blocking sidebar, rename, or account controls
• new installations include five additional ready-to-use employee profiles and their seeded resources
• existing SQLite installations retain noncanonical knowledge-base version identifiers safely
• the current login-page layout remains unchanged

Validation

• python -m pytest backend/tests/test_desktop_launcher.py backend/tests/test_auth_roles.py backend/tests/test_staffdeck_seed.py backend/tests/test_database_config.py (64 passed)
• npm --prefix frontend-enterprise test -- --run src/pages/LoginPage.test.tsx src/employee.test.ts (10 passed)
• npm --prefix frontend-enterprise run build
• npm --prefix frontend-enterprise run i18n:check
• npm --prefix frontend-enterprise run config:check
• built a macOS arm64 StaffDeck.app with PyInstaller
• packaged executable import smoke test passed
• packaged app started successfully and passed /api/health
• browser verification covered the restored landing page, admin/admin login, and all five added preset employees

[coder-zkl1988]

[coder-zkl1988]