Run library, checks and downloads in background threads with parallel fetching

src/downloader.py

@@ -99,9 +99,18 @@ class InstallerDownloader:
                 callback.on_finished(False, f"HTTP {response.status_code}")
             return False
 
-        total_size = int(response.headers.get("content-length", 0)) + existing_size
-        downloaded = existing_size
-        mode = "ab" if existing_size > 0 else "wb"
+        # Only append if the server honored the Range request (206). A plain 200
+        # means the body starts from byte 0, so appending would corrupt the file.
+        if existing_size > 0 and response.status_code == 206:
+            downloaded = existing_size
+            mode = "ab"
+            total_size = int(response.headers.get("content-length", 0)) + existing_size
+        else:
+            if existing_size > 0:
+                logger.warning(f"Server ignored Range for {real_filename}, restarting download")
+            downloaded = 0
+            mode = "wb"
+            total_size = int(response.headers.get("content-length", 0))
 
         last_time = time.monotonic()
         last_downloaded = downloaded
@@ -194,9 +203,17 @@ class InstallerDownloader:
                 callback.on_finished(False, f"HTTP {response.status_code}")
             return False
 
-        total_size = int(response.headers.get("content-length", 0)) + existing_size
-        downloaded = existing_size
-        mode = "ab" if existing_size > 0 else "wb"
+        # Only append if the server honored the Range request (206).
+        if existing_size > 0 and response.status_code == 206:
+            downloaded = existing_size
+            mode = "ab"
+            total_size = int(response.headers.get("content-length", 0)) + existing_size
+        else:
+            if existing_size > 0:
+                logger.warning(f"Server ignored Range for {real_filename}, restarting download")
+            downloaded = 0
+            mode = "wb"
+            total_size = int(response.headers.get("content-length", 0))
 
         last_time = time.monotonic()
         last_downloaded = downloaded