test(desktop): stabilize keyboard community reorder

[kiranmagic7]

Summary

Synchronize the keyboard-reorder test with dnd-kit's observable drag lifecycle. KeyboardSensor attaches its document keydown listener on the next timer tick, so the test could send ArrowUp before the sensor was ready. The drop could also race the updated collision target and preserve the original order.

The test now waits for pickup and for B to move above A before dropping. Product behavior is unchanged.

Related issue

Fixes #4530.

Testing

• On unmodified upstream/main, the issue reproduction produced 6 first-attempt failures out of 20; all retries passed.

• With this patch, the same command passed 20/20 without retries:

  cd desktop
  CI=1 pnpm exec playwright test tests/e2e/community-rail.spec.ts \
    --project=smoke \
    --grep 'keyboard reorder' \
    --repeat-each=20 \
    --reporter=list

• just ci

• Screenshots are not applicable; this is a test-only change.