[mirfalltnixein]

Fixes #361

A config file that ConfigParser can't parse, such as one with github_api_token set twice, crashes ProtonUp-Qt on startup at the first config read (apply_dark_theme → config_theme → read_update_config_value).

This follows the plan from the issue: before that first read, main() calls a new check_config_file(). If the file can't be parsed, it shows a warning with the parser's message (which names the option or section, the file and the line) and two buttons:

• Delete config (default): deletes the file and starts with the default settings.
• Exit, I'll fix it myself: exits and leaves the file alone. Closing the dialog does the same, so nothing is deleted by accident.

The check runs once at startup, so the three places that read the config (read_update_config_value, install_directory, config_custom_install_location) stay unchanged. It also covers the other parser errors, such as a duplicated section or a missing section header.

Testing:

• New parametrized test for get_config_file_error (missing file, valid file, duplicate option, duplicate section, no section header). No Qt dialog test, after the feedback on Batch update: match Proton tool names case-insensitively #662.
• QT_QPA_PLATFORM=offscreen pytest -q: 172 passed.
• Ran it from source with XDG_CONFIG_HOME pointing at a config with a duplicated github_api_token. Exit: the dialog appeared, ProtonUp-Qt exited and the file was kept. Delete config: the main window opened and a fresh default config was written.

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