fix: resume interrupted dumps by checking existing file size

[Ghostedworld]

Problem

When a dump is interrupted (power loss, crash, etc.), restarting
requires dumping the entire game from scratch even though most files
were already copied to USB.

The existing file-exists check in pfs.cpp had an uninitialized
variable bug — bytes was used before being assigned, causing
incorrect progress accounting.

Fix

• If a file already exists on USB and matches the expected size → skip it
• If a file exists but is smaller than expected (partial write) → delete and redo just that file
• All previously completed files are skipped correctly on resume

Result

Interrupted dumps resume from where they stopped instead of
restarting from 0%.

Note

Untested on hardware due to lack of build environment.
Logic reviewed carefully — looking for someone to verify on device.