feat(observability): add centralized diagnostics logging engine and native log viewer panel

- Configure tauri-plugin-log with 10MB rotation and 3-file retention
- Add high-frequency format filter to strip [download]% progress ticks
- Hook yt-dlp, aria2c, and native reqwest runners with lifecycle log macros
- Add export_logs Tauri command for async log file export via save dialog
- Create DiagnosticsView React component with streaming monospace console
- Apply severity highlighting: ERROR=red, WARN=yellow, INFO=grey, DEBUG=dim
- Wire Diagnostics tab into sidebar navigation and ActiveView routing
- Log native download failures to log::error! in download.rs

src/ipc.ts

@@ -100,6 +100,7 @@ type CommandMap = {
   db_save_queue: { args: { id: string; data: string }; result: void };
   db_delete_queue: { args: { id: string }; result: void };
   create_category_directories: { args: { paths: string[] }; result: void };
+  export_logs: { args: { destPath: string }; result: string };
   get_pending_order: { args: undefined; result: string[] };
   enqueue_download: { args: { item: any }; result: string };
   enqueue_many: { args: { items: any[] }; result: void };