[V1.3.3] Message Catalogue and Localization

[ahmad-ajmal]

Both error-mapping blocks (Gemini + OpenAI) are the same five patterns repeated with provider-specific text spliced in:

Template key	Triggers (substrings)	Placeholders
provider_rate_limit	quota, rate, insufficient_quota, billing	{provider_label}, {error}
provider_invalid_key	invalid + key, invalid_api_key	{provider_label}, {error}
provider_access_denied	permission, access	{provider_label}, {model}, {error}
provider_safety_block	safety, blocked, content_policy	{provider_label}, {error}
provider_model_not_found	not found, 404	{provider_label}, {model}, {help_url}, {error}

Suggested shape - anchors on the existing get_os_language() setting in app/config.py:341, which currently isn't wired to anything:

• app/i18n/errors.en.json - flat catalog of templates with {placeholder} substitution
• app/i18n/errors.<lang>.json - locale overrides; missing keys fall back to en
• app/i18n/__init__.py exposing two helpers:
  • t(key, **kwargs) -> str - formatted lookup with locale + en fallback
  • classify_provider_error(exc, *, provider, model) -> str - does the substring matching against a triggers table in the catalog and returns the formatted message in one call

The error block in this PR collapses to roughly:

except Exception as e:
    return {
        "status": "error",
        ...,
        "message": classify_provider_error(e, provider=provider, model=model_id),
    }

Benefits:

• one mapping table instead of two near-identical blocks per provider
• localization-ready without touching call sites
• adding a new provider = adding a row to the catalog, not another if/elif ladder
• avoids writing a bespoke message for every case - templates cover ~90% of them
• i think with the help of claude we can implement this pretty quick and easy