e2e: groups flow spec (create / manage / join via invite)

[chiptus]

This was generated by AI during triage.

Slice of the e2e coverage epic #106, following the pattern proven by #111 (voting spec, landed in PR #150).

Goal

Playwright e2e spec for the group lifecycle against the existing local-Supabase harness.

Behaviors to cover

• A signed-in user can create a group and sees it listed.
• The creator can manage the group (rename; see the member list; access the invite code/link).
• A second user can join via the invite code/link and then sees the group.
• A non-member cannot see group-scoped data (e.g. group notes/votes visibility) — assert the negative where cheaply possible.
• Leaving a group removes it from the user's list.

Harness & preconditions

• Build on the existing Playwright setup and TestHelpers; follow the suite's semantic-selector conventions, adding minimal data-testids to group controls only if needed.
• Confirm the seed provides two distinct test users (or a way to sign in as a second user); extend supabase/seed.sql if not — no production-synced data.
• If tests/README.md is still stale ("Boom Voter", port :5173), refresh it in passing.

Verification

• pnpm run test:setup then pnpm run test:e2e green locally; new spec deterministic across repeated runs.

Out of scope

Voting (done, #111), schedule/reveal, admin, set-detail — separate slices under #106.