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/App.tsx

@@ -14,6 +14,7 @@ import { useSettingsStore } from "./store/useSettingsStore";
 import { isPermissionGranted, requestPermission, sendNotification } from '@tauri-apps/plugin-notification';
 import SchedulerView from "./components/SchedulerView";
 import SpeedLimiterView from "./components/SpeedLimiterView";
+import DiagnosticsView from "./components/DiagnosticsView";
 
 function App() {
   const [filter, setFilter] = useState<SidebarFilter>('all');
@@ -305,6 +306,7 @@ function App() {
           {activeView === 'settings' && <SettingsView />}
           {activeView === 'scheduler' && <SchedulerView />}
           {activeView === 'speedLimiter' && <SpeedLimiterView />}
+          {activeView === 'diagnostics' && <DiagnosticsView />}
         </div>
         
         {/* Status Bar */}