feat(oled): add safer sleep and idle status carousel

[SiYue-ZO]

Summary

• Validate OLED sleep values and persist them atomically. Missing configuration now uses a 60-second default, while an explicit value of 0 still disables automatic sleep.
• Restart the idle countdown and clear carousel deadlines whenever the sleep setting changes.
• Use monotonic timing for OLED inactivity, carousel, event-wake, Wi-Fi, and button timers so NTP synchronization or wall-clock changes cannot trigger immediate carousel or sleep.
• Separate OLED power state from UI subpages. Sleep disables the panel and charge pump, while wake-up renders a complete frame before enabling the display.
• Make the main-page button a stable manual power toggle and bound hardware-event wake windows. Wi-Fi provisioning preserves its workflow state while applying an independent five-minute display timeout.
• Add a Cube idle dashboard with network, video, and session pages. The Never, 30-minute, and one-hour modes enter the carousel after 15 minutes of local inactivity. Pages rotate every six seconds and periodically shift position to reduce fixed-pixel exposure. Never continues rotating without automatically turning the display off.
• Publish active MJPEG, Direct, and WebRTC viewer counts through a two-second tmpfs heartbeat. Missing or stale state falls back to zero after five seconds, and short-lived capture leases are excluded.

Compatibility

• The OLED API remains unchanged: POST /api/vm/oled accepts sleep, and GET /api/vm/oled returns exist and sleep.
• Sleep values are restricted to 0 or 10–3600 seconds on both the server and kvm_system sides.
• Existing valid persisted values, including an explicit 0, are preserved.
• Devices without /etc/kvm/oled_sleep now use a 60-second default. This is an intentional behavior change.
• PCIe devices retain the legacy layout and do not use the Cube carousel or pixel offsets.
• Devices without an OLED continue to skip the OLED thread.
• Viewer heartbeat state is runtime-only under /run/nanokvm; missing or stale state is treated as zero.

Related to #835.
Partially addresses #105.