Add offline detection, chunked uploads, and security tests (#9, #10, #12, #15)

- Add offline detection with navigator.onLine and auto-pause/resume
- Implement chunked upload for files >100MB with localStorage persistence
- Add download by artifact ID input field in PackagePage
- Add 10 security tests for path traversal and malformed requests
- Fix test setup globalThis reference

frontend/src/test/setup.ts

@@ -18,7 +18,7 @@ class MockDataTransfer implements DataTransfer {
   setDragImage(): void {}
 }
 
-Object.defineProperty(global, 'DataTransfer', {
+Object.defineProperty(globalThis, 'DataTransfer', {
   value: MockDataTransfer,
 });