Add switchable light mode

[chrxh]

Summary

Adds a light mode that can be switched on and off via Settings > Light mode. The choice takes effect immediately and is persisted under settings.light mode.

Changes

• Theme dependent colors in the Const palette are no longer const. StyleService::setupPalette() assigns them with the dark and the light value side by side in HSV. The dark values are unchanged, so the dark mode looks exactly as before.
• A theme change is applied in StyleService::process() right before ImGui::NewFrame(). Changing the style during a frame would be reverted by pending PopStyleColor calls.
• New palette colors replace a few hardcoded colors: dashboard cards and summary row, neural net editor labels and node fill, and the own reaction frame in the browser. TextStrongColor replaces pure white text.
• The simulation view and everything drawn on top of it, the genome preview and the dashboard plot areas intentionally stay dark.

Known limitations

• The brief highlight of tree nodes brightens the header and is barely visible in light mode.
• Windows are 90% opaque, so light windows look slightly gray on top of the dark simulation.
• The colored checkbox backgrounds, for example in mass operations, stay dark.

Testing

• Release build
• DataTests, EngineInterfaceTests, NetworkTests and PersisterTests pass