fix: 性能优化、前端转写器配置、任务进度丢失及 MLX Whisper 回退问题修复

### 性能优化
- 后端任务执行从串行锁改为 ThreadPoolExecutor 并发执行（默认3线程）
- 添加 GZipMiddleware 响应压缩 + Nginx gzip 配置
- 数据库连接池参数优化（pool_size=10, max_overflow=20）
- 视频帧提取并行化（ThreadPoolExecutor）
- LLM 重试配置缓存到实例，避免每次请求读 env var
- 前端路由级代码拆分（React.lazy + Suspense）
- Vite manualChunks 拆分 markdown/markmap/vendor
- MarkdownViewer 用 React.memo + useMemo 减少不必要渲染
- NoteHistory Fuse.js 实例 useMemo 缓存
- useTaskPolling 无待处理任务时跳过轮询
- 移除 antd 依赖（NoteForm Alert、modelForm Tag），改用 shadcn/ui

### 前端转写器配置（新功能）
- 新增 TranscriberConfigManager（JSON 文件存储，替代环境变量）
- 新增 GET/POST /transcriber_config API 端点
- 新增 GET /transcriber_models_status 模型下载状态查询
- 新增 POST /transcriber_download 后台模型下载触发
- 前端转写器设置页面：引擎选择、模型大小选择、模型下载管理
- deploy_status 端点同步从配置文件读取

### Bug 修复
- 修复任务进行中切换页面后进度丢失：Home.tsx status 派生逻辑补全中间状态
- 修复 MLX Whisper 静默回退 fast-whisper：移除环境变量门控，macOS 下自动尝试导入
- MLX Whisper 不可用时抛出 RuntimeError 而非静默回退
- 前端展示 MLX Whisper 可用性状态，不可用时禁用保存

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

CLAUDE.md

@@ -0,0 +1,74 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+BiliNote is an AI video note generation tool. It extracts content from video links (Bilibili, YouTube, Douyin, Kuaishou, local files) and generates structured Markdown notes using LLM models. Full-stack app with a FastAPI backend, React frontend, and optional Tauri desktop packaging.
+
+## Development Commands
+
+### Backend (Python 3.11 + FastAPI)
+```bash
+cd backend
+pip install -r requirements.txt
+python main.py                    # Starts on 0.0.0.0:8483
+```
+
+### Frontend (React 19 + Vite + TypeScript)
+```bash
+cd BillNote_frontend
+pnpm install
+pnpm dev          # Dev server on port 3015, proxies /api to backend
+pnpm build        # Production build
+pnpm lint         # ESLint
+```
+
+### Docker
+```bash
+docker-compose up                              # Web stack (backend + frontend + nginx)
+docker-compose -f docker-compose.gpu.yml up    # GPU variant
+```
+
+### Desktop (Tauri)
+```bash
+cd backend && ./build.sh          # Build PyInstaller backend binary
+cd BillNote_frontend && pnpm tauri build
+```
+
+## Architecture
+
+**Backend** (`backend/`) — FastAPI app, entry point `main.py`:
+- `app/routers/` — API routes: `note.py` (generation), `provider.py`, `model.py`, `config.py`
+- `app/services/` — Business logic: `note.py` (NoteGenerator orchestrates the full pipeline), `task_serial_executor.py` (task queue)
+- `app/downloaders/` — Platform adapters (bilibili, youtube, douyin, kuaishou, local) with shared `base.py` interface
+- `app/transcriber/` — Speech-to-text engines (fast-whisper, groq, bcut, kuaishou, mlx-whisper) with factory in `transcriber_provider.py`
+- `app/gpt/` — LLM integration with factory pattern (`gpt_factory.py`), prompt templates (`prompt.py`, `prompt_builder.py`), and `request_chunker.py` for long transcripts
+- `app/db/` — SQLite + SQLAlchemy: DAO pattern (`provider_dao.py`, `model_dao.py`, `video_task_dao.py`), models in `models/`
+- `app/utils/` — `response.py` (ResponseWrapper for consistent JSON), `video_helper.py` (screenshots via FFmpeg), `export.py` (PDF/DOCX)
+- `events/` (root level) — Blinker signal system for post-processing (e.g., temp file cleanup after transcription)
+
+**Frontend** (`BillNote_frontend/src/`) — React 19 + Vite + Tailwind + shadcn/ui:
+- `pages/HomePage/` — Main note generation UI: `NoteForm.tsx` (input), `MarkdownViewer.tsx` (preview), `MarkmapComponent.tsx` (mind map)
+- `pages/SettingPage/` — LLM provider management, system monitoring, transcriber config
+- `store/` — Zustand stores: `taskStore`, `modelStore`, `configStore`, `providerStore`
+- `services/` — Axios API clients matching backend routes
+- `hooks/useTaskPolling.ts` — Polls task status every 3 seconds
+- `components/ui/` — shadcn/ui (Radix-based) components
+- Path alias: `@` → `./src`
+
+**Core Workflow**: User submits URL → task queued → download video → extract audio (FFmpeg) → transcribe (Whisper/Groq/etc) → generate notes (LLM) → frontend polls for completion → display Markdown + mind map.
+
+## Key Configuration
+
+- **Ports**: Backend 8483, Frontend dev 3015, Docker maps 3015→80
+- **Environment**: Root `.env` (copy from `.env.example`). LLM API keys are configured through the UI, not env vars.
+- **Database**: SQLite at `backend/app/db/bili_note.db`, auto-initialized on first run
+- **FFmpeg**: Required system dependency for video/audio processing
+- **Vite proxy**: Dev server proxies `/api` and `/static` to backend (configured in `vite.config.ts`, reads env from parent dir)
+
+## Code Style
+
+- **Frontend**: ESLint + Prettier (2 spaces, single quotes, 100 char width, Tailwind plugin). TypeScript strict mode.
+- **Backend**: Python with type hints. No configured linter. Uses Pydantic models for validation.
+- **Note**: The frontend directory is named `BillNote_frontend` (not "Bili").