Fix file/dir pickers, non-recursive filename scan, build hash in bundle names

[davidstephengrant]

Summary

• Stale path reset in file/dir pickers: When a stored path has become invalid, clicking to open a file or directory picker now resets that field immediately and falls back to the user's home directory, rather than letting the OS dialog open at an unreachable location. Applies to both drop-zone executable pickers and all three directory pickers (workdir, vtests, testfiles).
• Non-recursive testfile name scanning: scan_testfile_names previously walked subdirectories recursively, but the vtests scripts only process files directly in the selected directory. Simplified to a flat read_dir over the top level, removing the recursive collect_files helper and the by_parent HashMap.
• Build hash in bundle file names: Added a build npm script that runs tauri build then renames bundle outputs to insert the git hash (and -dirty flag if applicable) between the version and arch fields — e.g. vtests-gui_0.1.0_5c468e9_amd64.AppImage. Uses the same hash/dirty logic as build.rs.

Test plan

• Set a path in a drop zone, delete the file, click the drop zone — confirm the field clears and the picker opens in the home directory
• Set a directory path (workdir/vtests/testfiles), delete the directory, click the button — confirm the field clears and the picker opens in the home directory
• Place test files in a directory with subdirectories containing invalid names — confirm Validate filenames only reports files in the top level, not in subdirectories
• Run npm run build — confirm the AppImage is renamed to include the current git hash

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