Fix secure erase directory and batch handling

[Borisserz]

Summary

• refuse directories in Secure Erase mode so their contents are never recursively deleted without overwrite
• preserve the batch-size cap while validating and skipping unsafe paths per item
• show shredder errors on the result screen instead of displaying unconditional success

Test Plan

• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer bash scripts/check-version-sync.sh
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift build
• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test (809 tests, 3 skipped, 0 failures)
• New RED/GREEN regression tests cover secure directories and mixed-validity batches
• Tested in running app (not just compilation)

Screenshots

Not included; the result view only adds inline error text and a warning state.

Checklist

• Code compiles with swift build
• Follows existing code style (Swift 6, actors, @observable)
• No force unwrapping added
• File operations go through SafetyGuard
• No telemetry or network calls added
• PR is focused on the related shredder fixes

Closes #157
Closes #158

[iliyami]

Important fix, verified: secure erase of a directory now throws before removeItem (previously it silently deleted the directory's contents with no overwrite, a fake secure erase). Batch now erases the safe subset and skips unsafe paths, each still validated by SafetyGuard; #112 symlink/throwing-overwrite guards intact. Merging. Thanks @Borisserz!