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-tauri/src/download.rs

@@ -189,7 +189,7 @@ impl CoordinatorEventSink {
     fn emit_failed(&self, id: Uuid, error: String) {
         match self {
             Self::Tauri(app_handle) => {
-                eprintln!("download {id} failed: {error}");
+                log::error!("native download {} failed: {}", id, error);
                 let _ = app_handle.emit("download-failed", id.to_string());
             }
             Self::Headless(event_tx) => {