Sanitize search queries and ensure config file exists

[hanfoo]

Motivation

• Prevent unescaped user input from breaking or injecting into search regular expressions and make configuration handling more robust when the config file is missing or corrupted.

Description

• Add escapeRegExp and apply it in processContent and formatResults to safely escape the search query used in regexes.
• Add ensureConfigFile to create a default config at configName, ensure parent directories exist, and recover from corrupted JSON by overwriting with defaults.
• Refactor initMenu to use ensureConfigFile and remove the previous fragile file-read/try-catch flow while preserving upgrade logic for older config shapes.
• Guard fs.watch(configName) on app ready with fs.existsSync(configName) so file-watching is only set when the config file exists.