refactor: complete backend rewrite for multi-protocol proxy

Major rewrite of the Go backend to support:
- Three API format imports: openai, anthropic, compatible
- Three protocol conversions: Chat Completions, Responses, Messages
- Hub-and-spoke architecture with Chat as intermediate format

Deleted:
- opencat.go (old entry)
- store/, team/, pkg/team/, pkg/store/ (old data layer)
- internal/model/, internal/consts/ (old types)
- internal/service/team/, internal/controller/team/ (old handlers)
- llm/ (removed LLM client library, pure proxy mode)
- dist/, assets/ (old build artifacts)

Added:
- internal/store/ — 9 GORM models + multi-DB support
- internal/pkg/ — crypto (AES-GCM), apikey, jwt, ratelimit, resp, tokenizer
- internal/channel/ — channel selection, weighted LB, health checks
- internal/proxy/convert/ — 6 protocol conversion functions + SSE streaming
- internal/proxy/ — gateway with request dispatch and upstream selection
- internal/usage/ — async usage recorder with batch writes
- internal/api/ — management API (auth, users, keys, channels, models)
- Makefile for build/test/deploy

Fixed API to match frontend expectations:
- Login response wraps token in { data: { token } }
- GET /api/profile route added
- Profile response wraps user in { code, data }
- Role returned as number (10=admin, 1=user)

frontend/src/components/common/LineSegmentFlow.vue

@@ -1,7 +1,7 @@
 <template>
     <!-- 组件根元素：相对定位，设置最大宽度、外边距、宽高比、背景渐变、内边距、圆角、阴影和溢出隐藏 -->
     <div
-        class="relative w-full max-w-4xl mx-auto my-10 aspect-[4/3] backdrop-blur-0 px-4 py-0 my-0 rounded-lg  overflow-hidden ">
+        class="relative w-full max-w-4xl mx-auto my-10 aspect-[16/10] sm:aspect-[4/3] backdrop-blur-0 rounded-lg overflow-hidden ">
         <!-- bg-gradient-to-br from-slate-50 to-orange-50 -->
         <!-- 中心图标容器 -->
         <div ref="centerElement" class="absolute top-1/2 left-1/2 -translate-x-1/2 -translate-y-1/2 z-20">