Add bandwidth statistics tab with live chart

- Add new Statistics tab between Downloads and Settings
- Implement real-time bandwidth chart using Canvas (60s history)
- Add session overview with 8 stats cards (speed, downloaded, files, packages, etc.)
- Add provider statistics with progress bars
- Add getSessionStats IPC endpoint
- Support dark/light theme in chart rendering

src/main/app-controller.ts

@@ -5,6 +5,7 @@ import {
   AppSettings,
   DuplicatePolicy,
   ParsedPackageInput,
+  SessionStats,
   StartConflictEntry,
   StartConflictResolutionResult,
   UiSnapshot,
@@ -225,6 +226,10 @@ export class AppController {
     return this.manager.importQueue(json);
   }
 
+  public getSessionStats(): SessionStats {
+    return this.manager.getSessionStats();
+  }
+
   public shutdown(): void {
     abortActiveUpdateDownload();
     this.manager.prepareForShutdown();