Add read-only file indicator and save protection

[eugenioenko]

When opening a file with restricted permissions (e.g., chmod 444), there is no visual indicator that the file is read-only. The file can be edited and saved without warning.

Expected behavior

• Show a read-only indicator in the tab title (lock icon or "(read-only)") or status bar
• Warn or block save for read-only files unless the user explicitly chooses to overwrite

Notes

• On Linux, atomic save (temp file + rename) can succeed even for 444 files when the containing directory is writable, so the user unknowingly overwrites a read-only file

From QA BUG-019.