Fix Clerk convex token 404 fallback

[codybmenefee]

Summary

• replace strict ConvexProviderWithClerk usage with a ConvexProviderWithAuth bridge
• request Clerk token via configured template only when VITE_CLERK_CONVEX_TEMPLATE is set
• gracefully fall back to default Clerk session token if template token call fails (including 404)
• document optional VITE_CLERK_CONVEX_TEMPLATE in app/.env.example

Why
Production was repeatedly hitting POST /v1/client/sessions/:id/tokens/convex with 404 when the template was not configured, which left Convex auth in a bad state and caused noisy auth churn.

Validation

• cd app && npx eslint src/main.tsx
• cd app && pnpm run build