The data directory holds the SQLite database whose `sites` table
stores api_key and api_user in plaintext. Previous behavior used
the inherited umask (typically 0o755), which leaves the dir
world-traversable on shared workstations and on networked home
dirs whose home is 0o755. Tighten to 0o700 unconditionally on
every data_dir() call so the fix is applied even when an older
version (or external tooling) left the directory loose.
Failures from filesystems that don't support chmod (some FUSE
mounts) are swallowed — better to keep working than refuse to
start. Windows: no-op, NTFS ACLs handle this separately.
behavior change from v0.2.5: ~/.local/share/booru-viewer is now
0o700 even if it was previously 0o755.
Audit-Ref: SECURITY_AUDIT.md finding #4
Severity: Medium