Prevent battery slider XAML startup crash

[onehoon]

Summary

• Prevent the production Battery Charge Limit slider from handling XAML initialization changes before the frontend is connected.
• Add a regression test that verifies the handler returns before touching the draft value or value label when _frontend is null.

Root cause

DevicePage is created directly by MainWindow. During Slider.Minimum initialization, WinUI may coerce the initial value and raise ValueChanged before later-named XAML elements have been created. The battery handler previously touched BatteryChargeLimitValueText without the existing frontend-startup guard, causing the UI process to fail during MainWindow.InitializeComponent().

Validation

• Focused startup-guard regression: 1/1 passed.
• Full test suite: 2737/2737 passed.
• Release build: 0 warnings, 0 errors.
• Physical MSI Claw validation remains manual.

Scope

This is a narrow UI startup hotfix. It does not change the battery hardware protocol, runtime lifecycle, persistence, IPC, or unrelated UI behavior.