fix(gfn): macOS frame skipping during stream

[zortos293]

This PR addresses frame skipping during streaming on macOS (OPEN-16) by adjusting jitter buffer targets for VideoToolbox's higher latency and adding platform-specific Electron settings.

Changes in opennow-stable/src/renderer/src/gfn/webrtcClient.ts

• Add isMacOS() detection helper using navigator.userAgent/navigator.platform
• Add macOS-specific jitter constants: VIDEO_BASE_JITTER_TARGET_MAC_MS = 20 and VIDEO_PRESSURE_JITTER_TARGET_MAC_MS = 38 (vs 12ms/30ms on other platforms)
• Update receiverLatencyTargets initialization to use macOS-aware values
• Update setDecoderPressureMode and resetDecoderRecoveryState to select macOS-specific targets when platformIsMac is true

Changes in opennow-stable/src/main/index.ts

• Add backgroundThrottling: false to webPreferences to fully suppress renderer throttling beyond CLI switches
• Add fullscreen sync on macOS (darwin platform) by handling enter-html-full-screen and leave-html-full-screen events to prevent compositor hiccups during transitions