ui(settings): fix nested SingleChildScrollView in PreferencesAppearanceSection #1650

Problem

In lib/features/settings/preferences_dialog.dart, the detail pane host (_buildPaneHost) already wraps every selected preference pane in a vertical SingleChildScrollView:

child: material.SingleChildScrollView(
  primary: false,
  padding: const material.EdgeInsets.symmetric(horizontal: 24, vertical: 16),
  child: selected ? _paneWidgets[cat] = _buildPane(cat, theme) : ...,
)

However, in lib/features/settings/preferences_appearance_section.dart (line 155), build() inside ListenableBuilder returns a second nested material.SingleChildScrollView(primary: false, child: material.Column(...)).

Impact

• Creates two nested Scrollable widgets with unconstrained vertical height.
• On desktop platforms (macOS/Linux/Windows), mouse wheel and trackpad scroll events conflict between the inner and outer scroll controllers.
• The outer scrollbar does not accurately represent the content scroll extent of the Appearance tab.

Solution

Remove the inner material.SingleChildScrollView in preferences_appearance_section.dart and return material.Column directly (matching the other preferences sections like SQL, DataGrid, Extensions, About).