Modernize settings and stabilize live sampling

[yvette-carlisle]

Summary

• modernize the native Settings window, permission flow, hotkey/menu behavior, and related settings specs
• keep live HUD/loupe/toolbar chrome on overlay layers while preserving z-order and signed launch behavior
• restore fast stationary color sampling and reject stale startup-prewarm stream frames before first capture

Validation

• cargo make fmt-check
• cargo make lint-swift
• cargo test -p rsnap-overlay reset_discards_cached_frames_and_rejects_retired_stream_frames
• cargo make lint-rust
• scripts/smoke/native-hud-follow-macos.sh
• cargo make test-macos-native-host-stage
• git diff --check