fix: AMD HEVC green screen — GPU detection, codec fallback, and HEVC compatibility mode

[Meganugger]

Summary

Fixes the green screen issue on AMD Polaris/Vega GPUs when using HEVC by adding GPU capability detection, automatic codec fallback, and a user-facing HEVC compatibility mode setting.

Added

• gpuDetect.ts — WebGL-based GPU vendor/renderer detection with AMD generation classification (Polaris, Vega, RDNA, etc.)
• hevcCompatMode setting for manual HEVC compatibility override
• HEVC compatibility mode toggle in SettingsPage with GPU info display
• Automatic HEVC → H.264 fallback for GPUs known to produce green screen artifacts

Updated

• webrtcClient.ts — codec selection now checks GPU compatibility before using HEVC
• App.tsx — HEVC compat mode state management
• settings.ts — added hevcCompatMode default
• gfn.ts — added hevcCompatMode to Settings interface

Fixed

• Green screen on AMD Polaris/Vega GPUs when HEVC codec is selected
• Codec fallback now applies transparently without user intervention on affected hardware

Validation Notes

Recommended smoke checks:

• On an AMD Polaris/Vega GPU, start a stream with HEVC and verify no green screen (should auto-fallback)
• Check Settings → Video for HEVC compatibility mode toggle and GPU detection info
• On NVIDIA or AMD RDNA+ GPUs, verify HEVC still works normally
• Toggle HEVC compat mode manually and verify codec switches to H.264