fix(push): follow-ups from the push repair feedback review (#2145) #2153

Follow-up to #2145 (LOW notes from the merge pass's final review; none blocked).

• src/components/PushHealthNotice.test.tsx ~52-86 matches 10_000, a copy of the unexported WORKER_READY_TIMEOUT_MS (src/lib/push.ts ~18); export it and match on it. it.each([false, true]) does not fill %s, so both cases share one name; use [[false], [true]] or explicit names.
• A dismissed missing can outlast a recovery no check can prove when the server never answers the status check (open LAN 403, older server): a second loss in the same page session stays hidden (Settings still shows it). End a missing dismissal when a check finds no fault and the browser holds a subscription.
• enableNotifications guesses the failure by error type (src/lib/useNotifications.ts ~18-21): a non-JSON 200 from a proxy or a failed unsubscribe() shows the Brave/device sentence. Classify by the step that failed.
• A retired endpoint (subscription_gone) is not kept as a repair-failure reason, so that case still flickers with nothing said; more likely after fix(push): keep a retired endpoint refused past 30 days and under another spelling, and forget it when its files cannot be written #2140.
• docs/guide.md ~335 could mention Brave's "Use Google services for push messaging".