feat(ui): refactor AppDetail layout and add download with save dialog

- Remove Tabs component, merge JS/CSS sections into single scrollable list
- Add sub-page navigation for code viewing with back button
- Put app name and buttons on same row to save vertical space
- Reduce DomainForm spacing (marginMD -> marginSM)
- Use FileCode/FileText icons to indicate JS/CSS file types
- Add save dialog for single file download with loading state
- Add IPC handlers: showSaveDialog, saveFileContent
- Fix duplicate file extension issue in download filenames
- Add i18n keys: backToList, downloadSuccess, downloadFailed, downloadAllSuccess

src/main/ipc-handlers.ts

@@ -4,7 +4,7 @@
  * Based on REQUIREMENTS.md:228-268
  */
 
-import { ipcMain } from "electron";
+import { ipcMain, dialog } from "electron";
 import { v4 as uuidv4 } from "uuid";
 import {
   saveDomain,
@@ -573,6 +573,39 @@ export function registerIpcHandlers(): void {
   registerGetLocale();
   registerSetLocale();
 
+  // Dialog
+  registerShowSaveDialog();
+  registerSaveFileContent();
+
   console.log("IPC handlers registered");
 }
 
+// ==================== Dialog IPC Handlers ====================
+
+/**
+ * Show save dialog
+ */
+function registerShowSaveDialog(): void {
+  handle<{ defaultPath?: string }, string | null>("showSaveDialog", async (params) => {
+    const result = await dialog.showSaveDialog({
+      defaultPath: params.defaultPath,
+      filters: [
+        { name: "JavaScript", extensions: ["js"] },
+        { name: "CSS", extensions: ["css"] },
+        { name: "All Files", extensions: ["*"] },
+      ],
+    });
+    return result.filePath || null;
+  });
+}
+
+/**
+ * Save file content to disk
+ */
+function registerSaveFileContent(): void {
+  handle<{ filePath: string; content: string }, void>("saveFileContent", async (params) => {
+    const fs = await import("fs");
+    const buffer = Buffer.from(params.content, "base64");
+    await fs.promises.writeFile(params.filePath, buffer);
+  });
+}