fix(review-ui): set default window size

[lynnswap]

Summary

• Set ReviewMonitor's fallback main window content size to 600 x 400 when no saved frame exists.
• Keep AppKit frame autosave restoration intact by applying the autosave name after the fallback size is configured.
• Add a shell test that uses an isolated frame autosave name to verify the no-saved-frame default size path.

Testing

• swift test --build-system swiftbuild --no-parallel
• xcodebuild test -project Tools/ReviewMonitor/ReviewMonitor.xcodeproj -scheme ReviewMonitor -destination 'platform=macOS,arch=arm64' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO