Fix dark-theme ComboBox contrast and flatten button appearance Preset ComboBoxes in both control panels drew near-white item text without a matching buttonFill/listFill, so they fell back to SceneryStack's default white chrome — near-invisible against the default (dark) color profile. Give them dark buttonFill/listFill from SolarSystemModelsColors, matching the pattern already used in OscillationsAndChaos. Also add SolarSystemModelsButtonOptions.ts with a FlatAppearanceStrategy bundle (mirroring RotatingSky's RotatingSkyButtonOptions.ts) and apply it to every RectangularPushButton, ResetAllButton, and TimeControlNode play/pause/step button so the sim uses a flat button look instead of SceneryStack's default 3-D bevel. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_018svZaRUmD44JWTQJLUYdmx#7