test(desktop): stabilize community validation route check

[kiranmagic7]

Summary

The community-switch test asserted that the remembered channel was absent from the URL immediately after clicking community B. The switch intentionally writes that channel route before recording B as active, so the negative assertion races the async switch: it can pass against the transient Home route or time out after the switch completes.

Wait for community B's active-community marker, then assert the remembered route. The later assertions still verify that failed live validation preserves the remembered destination and a successful authoritative read repairs it to Home.

Related issue

Addresses the separate :716 failure signature in #6007. #4652 covers only the keyboard-reorder failure from the same issue. No existing pull request covers this route-validation failure.

Testing

• With the completed-switch wait and the existing negative URL assertion, 5/5 runs failed at that assertion.
• With the corrected positive assertion, 60/60 runs passed with retries disabled:
  PLAYWRIGHT_JSON_OUTPUT_NAME=playwright-results.json CI=1 pnpm --dir desktop exec playwright test tests/e2e/community-rail.spec.ts --project=smoke --grep 'does not repair a remembered channel until live validation succeeds' --repeat-each=60 --workers=1 --retries=0 --max-failures=0 --reporter=json
• pnpm --dir desktop exec biome check tests/e2e/community-rail.spec.ts
• pnpm --dir desktop typecheck
• CHECK_FILE_SIZES_BASE=upstream/main just ci
• Screenshots: N/A (test-only change)