feat(desktop): redesign login and improve macOS window chrome

[coder-zkl1988]

Summary

• redesign the login page around a centered form with responsive digital-employee illustrations
• allow a unique tenant-scoped display name to be used as a login identifier while preserving username priority
• remove the default macOS title bar, keep traffic lights integrated with the page, and provide a larger safe drag and double-click zoom region
• move sidebar branding and chat header controls below the native drag area

Why

The previous login experience was visually inconsistent with StaffDeck employee artwork, ordinary users could not sign in with their displayed account name after an administrator changed their password, and the titlebarless desktop shell had a very small drag target with overlapping top-left branding.

User impact

• clearer responsive login experience across large desktop, standard window, and narrow layouts
• ordinary members can sign in with a unique display name without weakening duplicate-name handling
• macOS users can drag and double-click zoom the titlebarless window without blocking sidebar, rename, or account controls

Validation

• backend/.venv/bin/python -m pytest backend/tests/test_desktop_launcher.py backend/tests/test_auth_roles.py backend/tests/test_database_config.py -q (53 passed)
• npm --prefix frontend-enterprise test -- src/pages/chat/components/ChatHeader.test.tsx src/components/AppSidebar.test.tsx src/pages/LoginPage.test.tsx (17 passed)
• npm --prefix frontend-enterprise run build
• packaged macOS arm64 smoke test and health check
• packaged-app interaction check: drag moved the window by 48x48 points and double-click zoom changed it from 1280x800 to 2237x1266
• packaged-app login check for a regular member using a unique display name

[coder-zkl1988]

[coder-zkl1988]