Fix: Show friendly message when invitation link has no id

[posthog-eu]

Problem

• A user who opens the invitation accept page without an id in the URL sees a full-app crash screen instead of guidance, in the signup and team-joining flow.
• apps/project/src/pages/invitations/accept.tsx did throw new Error("id not supplied") in the render path. Nothing caught it locally, so it reached the top-level ErrorBoundary in apps/project/src/main.tsx, which swaps the whole app for a raw ErrorAlert.
• The same component already handles bad invites gracefully (NTFND, DNIED, LOGIN). The missing-id case just never got the same treatment.

Changes

• Replace the throw with an inline "this invitation link is not valid" state, matching the existing NTFND copy, plus a link back to the app.
• Render it inside the normal SharedLayout (navbar + footer), so the page stays intact.
• No other change needed: the query is already paused via pause: !id, so this path makes no backend call.

Verification

• tsc -b and eslint pass for the project app.
• Loaded /invitations/accept with no query string in a browser: the page now shows the friendly message and a "Back to the app" button, with no crash.

Before: throw → top-level ErrorBoundary → whole app replaced by ErrorAlert.
After: friendly inline state within the normal layout.

---

Created with PostHog Desktop from this inbox report.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}