Fix disposal of pending account subscriptions

[ndisidore]

Previously, effect cleanup waited for the subscription RPC to resolve before disposing its stub. If the component unmounted while that RPC was still pending, the pending subscription could remain undisposed and leak server-side resources.

Cap’n Web RPC promises are themselves disposable, so this change disposes the pending promise directly during cleanup. This handles both pending and resolved subscriptions correctly.

[ask-bonk]

gatekeepers.tsx correctly removed the useRef import (no longer used), while GatekeeperModal.tsx keeps it because other refs remain. The import cleanup is correct — this matters since unused imports are lint errors in this repo.

The PR is clean and correct:

• The core fix is sound: Cap'n Web RPC promises are disposable, so disposing the promise directly handles both the pending and resolved cases, closing the leak where a component unmounts before subscribeConnectedAccounts resolves.
• The refactor is applied consistently across all six subscription sites, removing the now-unnecessary refs/local stub variables and the useRef import where it became unused.
• ObserverConfigModal.tsx correctly gains if (cancelled) return guards in its subscriber callbacks — previously the cancellation check only lived in the resolved .then, so with synchronous disposal those guards are needed to avoid setState-after-unmount. The other files already had equivalent guards.
• The new test properly exercises the pending-subscription path with a never-resolving promise carrying a [Symbol.dispose], and the fakeApi update to return a disposable resolved promise keeps existing tests accurate.

LGTM!

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