feat: add idle host display setting

[adamshiervani]

Closes #901, #975, #233

Summary

• Hide JetKVM's host display when no remote session is active, then restore it on reconnect.
• Add a video setting and JSON-RPC methods to control the behavior.
• Cover the idle display flow in remote-agent e2e tests.

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Note

Medium Risk
Changes HDMI EDID/HPD and session lifecycle behavior on the host; regressions could affect display enumeration, EDID presets, or reconnect UX, though logic includes rollback and e2e coverage.

Overview
Adds an optional hide JetKVM display when idle flow: when enabled and no remote WebRTC sessions are active, the device stops advertising HDMI EDID (clears EDID / drops HPD) so the host no longer sees JetKVM as a monitor; EDID is restored when the first session connects, on native restart, and when EDID or idle settings change.

Backend wires this through HideDisplayWhenIdle config, session hooks in webrtc.go, centralized applyHostDisplayAdvertisement / reapplyHostDisplayAdvertisement in video.go, a DisabledEDID sentinel, zero-size EDID support in native/CGO, and JSON-RPC getHostDisplayIdleMode / setHostDisplayIdleMode with safer setEDID rollback. The video settings UI adds a checkbox; WebRTC video adds a short startup grace before showing HDMI errors after hotplug cycles.

E2E covers host connector visibility across idle vs active sessions; strings added for all locales.

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