Battery- and thermal-aware auto-downgrade (snapshot-first / reserve mode)

[rayl15]

Part of the power-management roadmap in #36.

Goal

Make OpenVision battery- and thermal-aware: automatically prefer lower-power behavior as battery drops or the device heats up.

Why

There is currently no reading of battery level or thermal state anywhere in the app. This is the highest-impact new capability from the proposal for real all-day wear.

Scope (first pass)

• Read UIDevice.current.batteryLevel / .batteryState and ProcessInfo.processInfo.thermalState
• At low battery / high thermal: prefer a single snapshot over live video, shorten inactivity timeouts, avoid continuous streaming without confirmation
• A "Reserve mode" (voice-first, camera only on explicit request) below a configurable battery threshold
• Thresholds configurable in Settings, not hard-coded

Out of scope for this issue: the full set of named power profiles (All-Day/Balanced/Performance) — those can follow once the sensing + auto-downgrade primitives exist.

Pointers

• OpenVision/Models/AppSettings.swift — add threshold settings
• OpenVision/Views/VoiceAgent/VoiceAgentView.swift — gate live-video / snapshot decisions