feat(web): add missing variables to web/.env.example

[GRACENOBLE]

Problem

web/.env.example is missing several variables that are referenced in the codebase. A developer cloning the repo for the first time has no way to discover these without reading source code.

Variables to add

• NEXT_PUBLIC_API_URL — backend base URL used by the tRPC client and fetch utilities
• SENTRY_ORG — Sentry organisation slug (used in next.config)
• SENTRY_PROJECT — Sentry project slug
• Any Firebase config vars not already present (NEXT_PUBLIC_FIREBASE_*)

Acceptance criteria

• All env vars referenced anywhere in web/ appear in web/.env.example with a safe placeholder and a short comment
• The web README or setup doc references web/.env.example

[GRACENOBLE]

Implemented and merged via PR #65.