refactor: 合并为单 crate，前端改为内嵌 React 控制台

合并 stash-cli / stash-core 为根目录单 crate（lib + bin），新增 React + Vite + TS 控制台替换 Askama 模板。
build.rs 用 walkdir 把 stash-web/dist 同步到 $OUT_DIR/web，由 include_dir! 编译期内嵌，产出单文件二进制。
路由改为 / 与 /assets/** 走内嵌前端，其余交给 ServeDir，缺失路径返回内置 404。同步更新依赖、.gitignore 与 README。

src/error.rs

@@ -0,0 +1,45 @@
+use axum::{
+    Json,
+    http::StatusCode,
+    response::{IntoResponse, Response},
+};
+use serde_json::json;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum StashError {
+    #[error("IO 错误: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("文件监听失败: {0}")]
+    Watch(#[from] notify::Error),
+
+    #[error("目录不存在或非法: {0}")]
+    InvalidDirectory(String),
+
+    #[error("网络地址解析失败: {0}")]
+    AddrParse(#[from] std::net::AddrParseError),
+
+    #[error("端口绑定失败: {0}")]
+    ServerBind(String),
+
+    #[error("未找到资源: {0}")]
+    NotFound(String),
+}
+
+impl IntoResponse for StashError {
+    fn into_response(self) -> Response {
+        let (status, err_msg) = match self {
+            StashError::NotFound(msg) => (StatusCode::NOT_FOUND, msg),
+            StashError::InvalidDirectory(msg) => (StatusCode::BAD_REQUEST, msg),
+            _ => (StatusCode::INTERNAL_SERVER_ERROR, self.to_string()),
+        };
+
+        let body = Json(json!({
+            "error": err_msg,
+            "status": status.as_u16(),
+        }));
+
+        (status, body).into_response()
+    }
+}