feat: Linux ARM video backend separation and decode backend override UI

[Meganugger]

Summary

Separates Linux ARM video features from the main video pipeline and adds a user-facing decode backend override setting, allowing users to select between available video decode backends (auto, VAAPI, V4L2, software) on Linux ARM devices.

Added

• Decode backend override setting (decodeBackend) with options: auto, vaapi, v4l2, software
• Decode backend selector UI in SettingsPage (visible only on Linux ARM)
• Platform detection for Linux ARM in settings and renderer
• IPC channel for querying platform/architecture info
• Backend-specific video pipeline configuration in webrtcClient

Updated

• webrtcClient.ts — decode backend selection logic based on user setting and platform
• SettingsPage.tsx — conditional decode backend UI section for Linux ARM
• App.tsx — platform detection and decode backend state management
• settings.ts — decode backend default and persistence
• gfn.ts — decode backend types in Settings interface
• index.ts / preload/index.ts / ipc.ts — platform info IPC channel

Validation Notes

Recommended smoke checks:

• On a Linux ARM device, verify decode backend selector appears in Settings → Video
• Switch between decode backends and start a stream to verify each works
• On non-ARM Linux or other OS, verify the backend selector is hidden
• Verify the auto default selects the optimal backend for the platform