feat: 新增 grok-4.6 目录、官方定价与请求路径支持

官方目录价：<200k 为 $2 / 缓存读取 $0.50 / $6，≥200k 全部 2 倍。
原先没有独立价卡，/models 宣称可配 reasoning 但请求路径会剥掉 effort。

- 目录与别名接入 grok-4.6 / grok-4.6-latest，默认文本模型仍为 grok-4.5
- 独立回退价卡含缓存读取价与 200k 长上下文倍率
- 保留 reasoning_effort；Chat→Responses 的 cache/vision 桥接同样识别 4.6

frontend/src/composables/__tests__/useModelWhitelist.spec.ts

@@ -46,6 +46,8 @@ describe('useModelWhitelist', () => {
   it('xAI 模型列表包含 Grok 4.5 官方模型和别名', () => {
     const models = getModelsByPlatform('grok')
 
+    expect(models).toContain('grok-4.6')
+    expect(models).toContain('grok-4.6-latest')
     expect(models).toContain('grok-4.5')
     expect(models).toContain('grok-4.5-latest')
     expect(models).toContain('grok-build-latest')

frontend/src/composables/useModelWhitelist.ts

@@ -136,6 +136,7 @@ const metaModels = [
 
 // xAI Grok
 const xaiModels = [
+  'grok-4.6',
   'grok-4.5',
   'grok-4.3',
   'grok-build-0.1',
@@ -147,6 +148,7 @@ const xaiModels = [
   'grok-4.20-multi-agent-latest',
   'grok-4.3-latest',
   'grok-latest',
+  'grok-4.6-latest',
   'grok-4.5-latest',
   'grok-build-latest',
   'composer-2.5',
@@ -303,6 +305,7 @@ const geminiPresetMappings = [
 ]
 
 const grokPresetMappings = [
+  { label: 'Grok 4.6', from: 'grok-4.6', to: 'grok-4.6', color: 'bg-slate-100 text-slate-700 hover:bg-slate-200 dark:bg-slate-800/50 dark:text-slate-300' },
   { label: 'Grok 4.5', from: 'grok-4.5', to: 'grok-4.5', color: 'bg-slate-100 text-slate-700 hover:bg-slate-200 dark:bg-slate-800/50 dark:text-slate-300' },
   { label: 'Grok 4.3', from: 'grok-4.3', to: 'grok-4.3', color: 'bg-slate-100 text-slate-700 hover:bg-slate-200 dark:bg-slate-800/50 dark:text-slate-300' },
   { label: 'Grok Latest', from: 'grok-latest', to: 'grok-4.5', color: 'bg-emerald-100 text-emerald-700 hover:bg-emerald-200 dark:bg-emerald-900/30 dark:text-emerald-400' },