Defer Emscripten window events during a frame

[relh]

What changed

Focus, blur, and resize events that arrive while Asyncify has suspended onFrame are queued until the frame returns. This keeps window callbacks from changing state in the middle of a frame.

Check

• nim c -d:emscripten examples/basic.nim with Emscripten 6.0.2

[treeform]

Closing this PR because its changes were already included in #185 ("Fix double click never firing on emscripten"), merged into master on August 6, 2026 as d6ee2c9.

Verified the complete patch against that merged commit and checked the current master implementation: frameDispatchActive guards onFrame, resize and focus callbacks are deferred while the frame is active, and the pending events are drained after the frame returns.

The fix has already landed through #185, so no separate merge is needed. Thanks for the contribution!