fix: stream media downloads to avoid OOM on large files

[barrydeen]

Summary

• MediaDownloader.downloadMedia loaded the entire response body into a ByteArray via response.body.bytes(), which OOMs on large files (e.g. movies) — crash surfaced inside Okio Segment allocation while OkHttp was still buffering HTTP/2 frames.
• Stream the response body directly into the MediaStore/FileOutputStream with copyTo, keeping only an 8 KB buffer resident.
• saveImageBytes is unchanged — only used for already-bounded decrypted images.

Test plan

• Download a small image from a post — still saves to Downloads
• Download a large video/movie — completes without OOM
• Download on pre-Q device path still works