revert: 移除拼音声母搜索，回到 6 位代码输入

拼音搜索的底层依赖（首次需爬沪深 A 股 5000 条全名单，几十次 TDX 协议
往返，慢机器几十秒到超时）太重，反复优化（按需加载/遮罩/单飞/预热）
都无法兼顾'不阻塞核心行情请求'与'首次可用'。用户决定放弃此功能，
回到简单稳定的 6 位代码输入。

回退 e2bf29e..2523605 共 6 个 commit 的全部改动：
- 删除 StockSearchInput / AppInitOverlay / useStockSearch
- 移除 pypinyin 依赖、/security/search-index 端点、lifespan 预热
- SymbolPicker / StocksPicker 恢复为纯 6 位代码输入
- README / CHANGELOG 同步回退

代码状态等同 v1.18.1（一键寻优多进程并发）发布后的干净基线

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "easy-tdx"
-version = "1.19.0"
+version = "1.18.1"
 description = "通达信 TCP 协议行情数据客户端，支持在线行情、离线数据读取与写入同步"
 readme = "README.md"
 requires-python = ">=3.10"
@@ -16,7 +16,7 @@ easy-tdx = "easy_tdx.cli:cli"  # cli/__init__.py exposes the click group
 [project.optional-dependencies]
 dev = ["pytest>=8.0", "pytest-asyncio>=0.23", "pytest-cov", "mypy>=1.9", "ruff>=0.4", "scipy>=1.10,<1.16", "httpx>=0.27"]
 science = ["scipy>=1.10,<1.16"]
-web = ["fastapi>=0.110,<1", "uvicorn[standard]>=0.29", "pypinyin>=0.50"]
+web = ["fastapi>=0.110,<1", "uvicorn[standard]>=0.29"]
 
 [tool.hatch.build.targets.wheel]
 packages = ["src/easy_tdx"]