feat: integrate Grok2 API as preset provider for image/video generation

- Add grok2 to PRESET_PROVIDERS with openai-compat gateway route
- Add grok-imagine-1.0 (image) and grok-imagine-1.0-video (video) preset models
- Extend OpenAIImageGenerateSize to support 1280x720 and 720x1280 resolutions
- Add grok2 to COMPATIBLE_PROVIDER_KEYS in router
- Add grok2 to VERIFIABLE_PROVIDER_KEYS for test-connection UI
- Show Base URL input and model tabs for grok2 provider
- Add i18n labels for Grok2 API compatibility layer badge
- Add GROK2_SETUP.md with configuration documentation

messages/en/apiConfig.json

@@ -9,6 +9,7 @@
   "connect": "Connect",
   "compatibilityLayerOpenAI": "OpenAI Compatible Layer",
   "compatibilityLayerGemini": "Gemini Compatible Layer",
+  "compatibilityLayerGrok2": "Grok2 API",
   "show": "Show",
   "hide": "Hide",
   "capability": "Models",

messages/zh/apiConfig.json

@@ -9,6 +9,7 @@
   "connect": "连接",
   "compatibilityLayerOpenAI": "OpenAI 兼容层",
   "compatibilityLayerGemini": "Gemini 兼容层",
+  "compatibilityLayerGrok2": "Grok2 API",
   "show": "显示",
   "hide": "隐藏",
   "capability": "能力",