[AlexAndBear]

Description

Remove the saved dark flag in automatic mode so the existing startup logic follows the current system preference. Preserve the flag for manual themes.

Repro: Light system appearance → select Dark → select Auto → reload. Previously, a dark background briefly appeared.

Related Issue

• Fixes Automatic theme briefly flashes a dark background on reload #3842

How Has This Been Tested?

• test environment: macOS, Node.js 24.4.1, Vitest/happy-dom.
• test case 1: Automatic mode clears saved flags and follows the current system preference.
• test case 2: Manual themes persist; without a saved flag, the initial background follows the system.
• validation: 58 affected unit tests, type check, lint and changed-file formatting passed.

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)