Changes

PR #2019 by ksylvan: feat: replace hardcoded error strings with i18n translation keys

• Replace hardcoded error messages with i18n.T() calls across Go source files, covering chat, attachment, storage, template, plugin registry, server, and utility packages
• Added approximately 80 new translation keys to all locale files (en, de, es, fa, fr, it, ja, pt-BR, pt-PT, zh), with keys sorted alphabetically
• Internationalized error strings across githelper, notifications, patterns, sessions, and provider modules (DigitalOcean, Gemini, OpenAI-compatible)
• Refactored fmt.Errorf("%s", ...) usages to errors.New() and normalized i18n error strings to lowercase per Go conventions
• Updated db_error_loading_env_file format verb from %s to %w to support proper error wrapping