2c4c6adb

Rename remaining fsys variables to clearer names

Finish the cleanup started in d60147a by replacing every remaining
generic fsys variable with a name that says what the filesystem holds:
sourceDirFS for the source-media directory read during resize/convert,
and exportFS for the Tinybeans export filesystem.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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