Fix lobby race: single-transaction gated add

[wasita]

Summary

• Collapse `reqMetaDocChange` add + capacity check + reset into a single Firestore transaction, closing the two-transaction gap that allowed `meta.counter` to overflow past `groupSize` and deadlock the lobby whenever 3+ users joined near-simultaneously.
• Gate the add on capacity so the invariant `counter.length < groupSize` holds after every committed write. Overflow becomes structurally impossible, and the `=== groupSize` check on the closing user is safe again.
• Return the dyad from the transaction callback instead of capturing it in a closure variable, so Firestore retries can't leak stale state across attempts.
• Throw `new Error(...)` instead of string literals; use a status-tagged return value (`duplicate` / `full` / `waiting` / `closed`) so `LOBBY_FULL` is a loud assertion rather than a silent deadlock.
• Delete `checkIfResetCounter` and the commented-out pairing loop that was never viable (non-transactional `initGroup` side effects).

How it works

Each `reqMetaDocChange` call runs one transaction that either:

• duplicate: user already in counter → no-op
• full: counter is already at capacity (should never fire under the new invariant; surfaced as a warning so external writes or bugs are visible)
• waiting: user appended, counter still below capacity
• closed: user appended, counter hit capacity, reset to `[]` atomically, dyad returned

Only the closed caller runs `initGroup`. Non-closing users are notified via the existing `onSnapshot` listener on their participant doc in `App.svelte:188`, which reactively subscribes them to the group doc once `initGroup` writes their `groupId` at `utils.js:1161`. (Listener chain verified in this PR.)

Test plan

• 2 browser windows: confirm the existing happy path still forms a dyad and both users advance.
• 3 browser windows opened nearly simultaneously: first two should pair, third should remain queued until a 4th joins.
• 4+ browser windows opened nearly simultaneously: two dyads form, no users stranded, no duplicate groups.
• Double-click / re-entry on a user already in the counter: dedupe path hits, no spurious state change.
• Manually set `meta.counter` to length `>= groupSize` in the Firebase console, then have a user join: `LOBBY_FULL` warning should fire and the user should not be added.

[github-actions]

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