checkNow() previously delegated to _runAll(), which gated each
account on the _running flag (only true after start() is called).
This meant the manual "Verify sync health" action silently did nothing
if start() had not yet been called, or in any context where the
periodic runner was not active (e.g. widget tests).
Fix: checkNow() now iterates accounts directly and calls
_runForAccount() with force:true, bypassing the _running guard.
The guard is still respected during periodic runs for graceful
shutdown.
Adds three unit tests that reproduce the bug and verify the fix.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
