Fix download resume corruption when server returns HTTP 200

When resuming a download after interruption, if the server returns
HTTP 200 (full resource) instead of HTTP 206 (partial content), the
code correctly resets mMission.done but fails to reset the 'start'
variable. This causes the subsequent file seek to use a stale offset,
writing new data at incorrect positions.

This bug causes file corruption for large downloads (>5GB) that are
interrupted and resumed, particularly when:
- Switching between WiFi networks
- Server CDN returning different responses
- Connection drops during long downloads

The corruption manifests as duplicate data regions in the file,
which for MP4 downloads results in multiple MOOV atoms and
broken seek functionality.

Fix: Reset start=0 when HTTP 200 is received, ensuring the file
write position correctly restarts from the beginning of the current
resource.

app/src/main/java/us/shandian/giga/get/DownloadRunnableFallback.java

@@ -85,6 +85,7 @@ public class DownloadRunnableFallback extends Thread {
             if (mMission.unknownLength || mConn.getResponseCode() == 200) {
                 // restart amount of bytes downloaded
                 mMission.done = mMission.offsets[mMission.current] - mMission.offsets[0];
+                start = 0; // reset position to avoid writing at wrong offset
             }
 
             mF = mMission.storage.getStream();