feat: gate remote projects as a technical preview

Remote Host and managed SSH project entry points were always exposed while the signed artifact workflow remains preview-stage. Remote Projects now defaults off behind its own Platform Features tab, preserves saved Hosts, projects, and credentials, and returns active remote work to a local project when disabled.

Main IPC enforces the same setting while still allowing in-flight cancellation. Application settings migrate to version 9 with the preview disabled.

Release note: 新增“远程项目（技术预览）”独立页签和开关，默认关闭；启用后显示 SSH Host 与远程项目功能，关闭不会删除已有配置。

src/shared/application-settings-contracts.ts

@@ -29,6 +29,7 @@ const applicationPreferencesSchema = z
     checkUpdatesOnStartup: z.boolean(),
     updateSource: updateSourceSchema,
     modelDownloadSource: modelDownloadSourceSchema,
+    remoteProjectsEnabled: z.boolean(),
     magicNotesEnabled: z.boolean(),
     magicNotesShowIncompleteTodoCount: z.boolean(),
     magicNoteCommentMode: magicNoteCommentModeSchema,