refactor: rewrite model-context to use js-yaml, add context_length to provider form (#177)

* fix: context-length API returns 200K instead of actual model context

Two bugs cause the /api/hermes/sessions/context-length endpoint to
always return DEFAULT_CONTEXT_LENGTH (200K):

1. getModelContextLength ignores config.yaml model.context_length
   The function only checks models_dev_cache.json (which doesn't
   exist in default installations) and falls back to the hardcoded
   200K default, completely ignoring the user's explicit
   model.context_length setting in config.yaml.

2. getDefaultModel regex fails when api_key/base_url come before default
   The regex /^model:\s*\n\s+default:\s*(.+)$/m assumes 'default' is
   the first child key under 'model:', but when api_key or base_url
   appear first in the YAML, the match fails. This causes
   getModelContextLength to short-circuit to DEFAULT_CONTEXT_LENGTH
   before even reaching the cache lookup.

Fix:
- Add getDefaultModelRobust() that extracts the entire model: block
  first, then searches for default: within it
- Add getConfigContextLength() that reads model.context_length from
  config.yaml as a fallback (matching hermes-agent priority)
- Update getModelContextLength() resolution order:
  1. models_dev_cache.json (existing)
  2. config.yaml model.context_length (new)
  3. DEFAULT_CONTEXT_LENGTH (existing fallback)

Closes #169

* refactor: rewrite model-context to use js-yaml, add context_length to provider form

- Replace fragile regex-based YAML parsing with js-yaml for reliable config.yaml reads
- Fix context_length resolution priority: config.yaml override > custom_providers > models_dev_cache > 200K default
- Add context_length input field when adding custom providers in ProviderFormModal
- Backend: persist context_length to custom_providers models.<model>.context_length in config.yaml
- Add i18n keys (contextLength, contextLengthPlaceholder) to all 8 locales

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: use NInputNumber instead of NInput type=number for context_length

NInput does not support type="number" in Naive UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: devilardis <53129661@qq.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/i18n/locales/pt.ts

@@ -259,6 +259,8 @@ export default {
     builtIn: 'Integrado',
     customType: 'Personalizado',
     provider: 'Provedor',
+    contextLength: 'Tamanho do contexto',
+    contextLengthPlaceholder: 'ex: 200000 (opcional)',
     local: 'Local ({host})',
     selectProviderRequired: 'Por favor, selecione um provedor',
     baseUrlRequired: 'A URL base e obrigatoria',