feat: add desktop notification toggle to general settings

Add a default-on persisted desktop notification switch that takes effect immediately for background task results while leaving in-app notifications unchanged. Group General settings into shortcut, conversation and notification, and workspace and download cards.

Validation: 756 related tests passed, 1 skipped; typecheck and lint passed. Full suite timed out with three supervision preview failures; native OS notification delivery was not manually verified.

Release note: 通用设置新增默认开启的桌面通知开关，可随时关闭系统任务提醒并保留应用内提示；设置项按用途分组，方便查找。

src/shared/application-settings-contracts.ts

@@ -58,6 +58,7 @@ export const applicationNavigationSchema = z.object({
 const applicationPreferencesSchema = z
   .object({
     checkUpdatesOnStartup: z.boolean(),
+    desktopNotificationsEnabled: z.boolean().default(true),
     updateSource: updateSourceSchema,
     modelDownloadSource: modelDownloadSourceSchema,
     localToolEnvironment: localToolEnvironmentSettingsSchema,
@@ -88,6 +89,7 @@ export const applicationSettingsUpdateSchema = applicationPreferencesSchema
   .partial()
   .extend({
     applicationNavigation: applicationNavigationSchema.optional(),
+    desktopNotificationsEnabled: z.boolean().optional(),
     localInferenceEnabled: z.boolean().optional(),
     magicNotesEnabled: z.boolean().optional(),
     heartbeatEnabled: z.boolean().optional(),