feat(settings): add a scrollbar minimap switch under Appearance

chatMinimapEnabled defaults to on; turning it off restores the native message scrollbar while keeping continuous history and DOM windowing.

src/shared/application-settings-contracts.ts

@@ -60,6 +60,7 @@ const applicationPreferencesSchema = z
     checkUpdatesOnStartup: z.boolean(),
     desktopNotificationsEnabled: z.boolean().default(true),
     transparentFrostedEffectEnabled: z.boolean().default(true),
+    chatMinimapEnabled: z.boolean().default(true),
     updateSource: updateSourceSchema,
     modelDownloadSource: modelDownloadSourceSchema,
     localToolEnvironment: localToolEnvironmentSettingsSchema,
@@ -94,6 +95,7 @@ export const applicationSettingsUpdateSchema = applicationPreferencesSchema
     applicationNavigation: applicationNavigationSchema.optional(),
     desktopNotificationsEnabled: z.boolean().optional(),
     transparentFrostedEffectEnabled: z.boolean().optional(),
+    chatMinimapEnabled: z.boolean().optional(),
     localInferenceEnabled: z.boolean().optional(),
     deviceSharingEnabled: z.boolean().optional(),
     magicNotesEnabled: z.boolean().optional(),