Please address the comments from this code review:

## Overall Comments
- The temporary extraction directory `temp_dir.join("mxu_webview2_extract")` is a fixed path; consider using a per-run unique directory (e.g., including PID or a random suffix) to avoid clashes or race conditions when multiple instances run concurrently or when a previous run left the directory behind.

## Individual Comments

### Comment 1
<location> `src-tauri/src/webview2/install.rs:97-99` </location>
<code_context>
+}
+
+/// 解压 cab 文件到 WebView2 运行时目录
+fn extract_cab_to_runtime(cab_path: &std::path::Path, runtime_dir: &std::path::Path) -> Result<(), String> {
+    let temp_dir = std::env::temp_dir();
+    let extract_temp = temp_dir.join("mxu_webview2_extract");
+
+    let _ = std::fs::remove_dir_all(&extract_temp);
</code_context>

<issue_to_address>
**issue (bug_risk):** Using a fixed temp directory for extraction can cause conflicts between multiple instances.

The extraction path is always `%TEMP%/mxu_webview2_extract`, which is deleted before each run. If multiple instances run concurrently or another process shares this helper, they could delete or overwrite each other’s temp directory, leading to intermittent failures or incomplete cleanup. Use a per-run unique temp directory (e.g., PID/timestamp/random suffix or a `tempfile`-style API) to avoid these races.
</issue_to_address>

### Comment 2
<location> `src-tauri/src/webview2/install.rs:184-193` </location>
<code_context>
+    }
+
+    // 优先使用架构匹配的 cab
+    if let Some(cab_path) = matched {
+        let progress_dialog = CustomDialog::new_progress(
+            "正在解压 WebView2",
+            "检测到本地 WebView2 运行时 cab 文件，正在解压...",
+        );
+
+        let result = extract_cab_to_runtime(&cab_path, runtime_dir);
+
+        if let Some(pw) = progress_dialog {
+            pw.close();
+        }
+
+        if result.is_ok() {
+            let _ = std::fs::remove_file(&cab_path);
+        }
+        return Some(result);
+    }
+
</code_context>

<issue_to_address>
**issue:** Local cab extraction failures do not fall back to online download as the comment suggests.

The doc for `try_extract_local_cab` says it should “返回 None 继续下载” when the local cab is unusable, but if a matching cab exists and `extract_cab_to_runtime` fails (e.g. corrupted/partial cab), this branch returns `Some(Err(_))`. `download_and_extract` then exits without trying the online download, so a single bad local cab can permanently block recovery. Consider, on extraction failure: (1) delete the cab, (2) return `None` to trigger the online download, and (3) only bubble up an error if the online download also fails.
</issue_to_address>

Please address the comments from this code review:

## Overall Comments
- The `extract_cab_to_runtime` helper assumes `expand.exe` is available on PATH and doesn’t exist or resolve failures beyond a generic error; consider resolving it explicitly from `System32` and surfacing a clearer message when it’s missing to avoid confusing failures on misconfigured environments.
- In `download_and_extract`, the temporary cab path is just `temp_dir.join(cab_name)`, so concurrent instances of the app could contend for the same file; including the process ID or a random suffix in the temp filename would make this more robust.

## Individual Comments

### Comment 1
<location> `src-tauri/src/webview2/install.rs:106-102` </location>
<code_context>
+fn extract_cab_to_runtime(cab_path: &std::path::Path, runtime_dir: &std::path::Path) -> Result<(), String> {
</code_context>

<issue_to_address>
**suggestion:** Consider hardening the `expand.exe` invocation and handling the case where `expand.exe` is missing or not on PATH.

This relies on `expand.exe` being on PATH; in locked-down or minimal Windows setups it may be missing, resulting in a generic “运行 expand.exe 失败” message.

I’d suggest:
- Resolving `%SystemRoot%\System32\expand.exe` (e.g., via `GetSystemDirectoryW`) instead of depending on PATH.
- Distinguishing between “executable not found” and a non‑zero exit code so the error clearly indicates when `expand.exe` itself is unavailable.

That will make cab extraction failures more diagnosable for users.
</issue_to_address>