Auth: Web dashboard — AuthContext + API interceptor

[EricGrill]

Auth System — Web Step 1

Add auth state management to the React web dashboard.

New files:

• web/src/contexts/AuthContext.tsx — user state, tokens, login/logout/refresh
• web/src/hooks/useAuth.ts — typed access to AuthContext
• web/src/hooks/useRequireAuth.ts — redirect to /login if unauthenticated

API interceptor (web/src/api.ts):

• Attach Authorization: Bearer <token> to all requests
• Intercept 401 → attempt silent refresh → retry or redirect to login
• Access token stored in React ref (memory only, never localStorage)
• Refresh token in HttpOnly cookie (set by backend)

On app mount:

• Attempt silent refresh (cookie sent automatically)
• Success → user logged in without seeing login screen
• Failure → redirect to /login

Farm context integration:

• On login, auto-select farm if user has exactly one
• Filter farm picker to user's assigned farms

Depends on: Backend auth endpoints
Design doc: docs/AUTH.md

[EricGrill]

Implemented in PR #140. JWT auth system deployed to production.