Extract SettingsPanel UserControl from MainWindow

[mock1ngbb]

UI MIGRATION — move ~600 lines of settings panel construction and event handlers into a dedicated UserControl.

Extract from MainWindow.axaml.cs:

• All settings-related event handlers (ThemeColorPicker_Click, SecondaryColorPicker_Click, ShowColorPresetsDialog, ShowCustomColorPicker, etc.)
• Settings panel controls building (sliders, checkboxes, textboxes)
• Token field UI (GitHubTokenTextBox + associated logic)

New files: Controls/SettingsPanel.axaml + Controls/SettingsPanel.axaml.cs

• Wire via <local:SettingsPanel /> tag in MainWindow.axaml

Verification: dotnet build passes, settings panel renders and all controls function.
Doable by: sonnet/cline-agent under 45 min.