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。

stash-web/package.json

@@ -0,0 +1,29 @@
+{
+  "name": "stash-web",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "oxlint",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "react": "^19.3.1",
+    "react-dom": "^19.3.1"
+  },
+  "devDependencies": {
+    "@types/node": "^24.19.2",
+    "@types/react": "^19.3.0",
+    "@types/react-dom": "^19.3.0",
+    "@vitejs/plugin-react": "^6.1.2",
+    "oxlint": "^1.88.0",
+    "typescript": "~6.0.3",
+    "vite": "^8.3.4"
+  },
+  "volta": {
+    "node": "24.12.0",
+    "npm": "11.6.2"
+  }
+}