SFTP: Stream file transfers instead of buffering entire file in memory

[BrianLeishman]

Summary

SFTP file operations (download_file_from_sftp, upload_file_to_sftp, copy) read entire files into a Vec<u8>, which will OOM on large files.

Current State

• src-tauri/src/locations/sftp/mod.rs:385 — sftp.read(remote_path) loads full file
• src-tauri/src/locations/sftp/mod.rs:408 — tokio::fs::read(local_path) loads full file
• src-tauri/src/locations/sftp/mod.rs:268 — copy() reads full file into memory
• Same pattern exists in SMB provider (via sidecar)

Proposed Changes

Use russh-sftp's open() + chunked read()/write() instead of the convenience read()/write() methods:

let file = sftp.open(path).await?;
let mut buf = vec![0u8; 64 * 1024]; // 64KB chunks
loop {
    let n = file.read(&mut buf).await?;
    if n == 0 { break; }
    dest.write_all(&buf[..n]).await?;
}

Acceptance Criteria

• File transfers use constant memory regardless of file size
• Downloads stream to disk in chunks
• Uploads stream from disk in chunks
• Copy streams through a temp file without full buffering
• Progress reporting for large transfers (nice-to-have)