Fix frontend auth success routing

[cursor]

Bug and impact

Successful login/signup crashed in the frontend because the success handler expected data.session.access_token, while the backend returns data.tokens.accessToken. Even if that crash was bypassed, auth redirected to /dashboard while the dashboard page lived in a route group that resolved to /, creating a broken post-auth path and a root route conflict.

Root cause

The frontend auth response type and hook were out of sync with the backend auth contract, and the dashboard route files were placed under app/(dashboard) instead of the /dashboard URL segment used by redirects and navigation.

Fix

• Updated frontend auth response typing and useAuth success handlers to read tokens.accessToken / tokens.refreshToken.
• Moved the dashboard layout, dashboard page, recipes page, and layout test to app/dashboard/... so /dashboard and /dashboard/recipes are real routes.
• Added regression tests for login/signup success token storage and dashboard navigation.

Validation

• pnpm --filter @snacktrack/frontend test -- src/lib/hooks/use-auth.test.tsx src/app/dashboard/layout.test.tsx -> 5 tests passed.
• pnpm --filter @snacktrack/shared-types build && pnpm --filter @snacktrack/frontend build -> production build succeeded and listed /dashboard and /dashboard/recipes routes.
• Manual browser walkthrough recorded for /dashboard -> /dashboard/recipes -> /dashboard routing.