fix(social) + test(e2e): the follows surface works + the groups e2e floor/gauntlet

[jacoby149]

What

The social app's follows surface (social-e2e lane, the M0 machine track) — follows as the app drives them: follow → the creator's posts enter the feed → unfollow → they leave.

The bug: the app's followersGroupId returned web10.app/groups/{username}/followers — the wrong shape (missing the users/ segment) and a hardcoded provider instead of the node's. The API derives a created group's ID from the token's provider claim as {provider}/groups/users/{creator}/{slug}, so the app was addressing a group that never exists: the follow button's joinGroup 404'd and silently reverted to "Follow".

The fix (src/data/groups.ts + follows.ts):

• followersGroupId(username, provider?) → {provider}/groups/users/{username}/followers (provider = the token's, falling back to API_HOST).
• ensureFollowers creates under the bare slug followers with the bare-username owner key (the format joins + discover auto-enrollment use).
• isFollowing checks getMyGroups() membership (the API's groups/get doesn't return my_role, so the old check was always false).
• The follow button gets [social] logging at each transition.

The e2e (e2e/tests/social-groups.spec.ts):

• API floor: the app's exact feed read (getMyGroups minus discover → one multi-group posts read) + the follow/unfollow group ops + the feed-read delta (follow → the creator's post enters the feed; unfollow → it's gone, 403 on a direct read) + the I3 anti-test (a stranger who never followed can't read the followers group).
• Browser gauntlet: viewer pre-authed via the token cookie → open the creator's /u/:username → follow (the real button) → the post appears in /feed → unfollow → it leaves, with console log-sequence verification.

Group management (create/roles/invite) stays the authenticator + marketing-directory surface — its floors live in groups-demo.

Verification

• scripts/run-e2e.sh tests/social-groups.spec.ts → 3/3 green (stable across runs)
• 200 web10-social unit tests green (follows.test.ts re-aligned to the new ID pattern)
• tsc -b + vite build clean; e2e spec tsc-clean (remaining e2e tsc errors are pre-existing on dev)
• Screenshots verified desktop + 375px (follow button Follow→Following, feed reflects it)