fix(web): restore login page design and add runtime config injection

Login page was deployed with the old unstyled version on main — the
Round 5 design system components (AuthShell, AuthCard, AuthBrand) had
never been merged. This commit restores the designed login experience
with animated gradient background, card chrome, and proper dark-mode
support matching the design reference.

Additionally, NEXT_PUBLIC_API_URL was hardcoded at build time via the
CI --build-arg, making it impossible to override in container env vars.
The root layout now injects runtime env vars into window.__MOSAIC_ENV__
via a synchronous script tag, and config.ts reads those values first.
This lets deployed containers use their own API URL without rebuilding.

Changes:
- Add AuthSurface.tsx to @mosaic/ui (AuthShell, AuthCard, AuthBrand,
  AuthStatusPill, AuthDivider)
- Rewrite login page to use design system components with "Command
  Center" heading matching the design reference
- Update config.ts with getEnv() helper that reads window.__MOSAIC_ENV__
- Add runtime env injection script to root layout.tsx
- Update all tests to match new component structure and content

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/components/auth/AuthErrorBanner.tsx

@@ -13,7 +13,7 @@ export function AuthErrorBanner({ message, onDismiss }: AuthErrorBannerProps): R
     <div
       role="alert"
       aria-live="polite"
-      className="bg-blue-50 border border-blue-200 text-blue-700 rounded-lg p-4 flex items-start gap-3"
+      className="flex items-start gap-3 rounded-lg border border-[#f06a6f]/55 bg-[#fff1f2] p-4 text-[#9f1239] dark:border-[#e5484d]/55 dark:bg-[#3a111b]/70 dark:text-[#fecdd3]"
     >
       <Info className="h-5 w-5 flex-shrink-0 mt-0.5" aria-hidden="true" />
       <span className="flex-1 text-sm">{message}</span>
@@ -21,7 +21,7 @@ export function AuthErrorBanner({ message, onDismiss }: AuthErrorBannerProps): R
         <button
           type="button"
           onClick={onDismiss}
-          className="flex-shrink-0 text-blue-500 hover:text-blue-700 transition-colors"
+          className="flex-shrink-0 text-[#be123c] transition-colors hover:text-[#881337] dark:text-[#fda4af] dark:hover:text-[#ffe4e6]"
           aria-label="Dismiss"
         >
           <X className="h-4 w-4" aria-hidden="true" />