feat: add frontend authentication and API foundation

[linuka7]

M20 authentication and API foundation

Completes the first authenticated frontend flow for DispatchArc.

Included

• development-only CORS for Vite localhost
• typed frontend API client and shared contracts
• auth/session storage using sessionStorage
• real login screen wired to POST /api/auth/login
• protected frontend routing
• authenticated user name and role in the dashboard shell
• logout flow and redirect protection
• Jobs API client foundation
• frontend environment example for API base URL

Verification

• real tenant + Owner account login tested end-to-end
• JWT/session flow verified in browser
• protected /dashboard redirect verified after logout
• frontend production build passed
• frontend lint passed
• backend test suite passed
• git diff whitespace check passed

This closes M20 and prepares M21 for the real Jobs workspace.