feat: web UI + HTTP server + Electron shell

- serve.go: embedded web UI (embed.FS), REST API endpoints for
  search, list, download (SSE progress), and WAV extraction
- main.go: added serve subcommand and flag registration
- web/ui: vanilla JS frontend with search cards, download queue,
  extract controls; dark theme, zero dependencies
- electron/: Electron wrapper that spawns fetch serve as sidecar,
  reads port from stdout, creates BrowserWindow; electron-builder
  config for macOS .app bundle with akaiutil + script as resources

Refs #1, #9

electron/main.js

@@ -0,0 +1,122 @@
+const { app, BrowserWindow, dialog } = require('electron');
+const path = require('path');
+const { spawn } = require('child_process');
+
+let mainWindow;
+let serverProcess;
+let serverPort = 0;
+
+function findServerBinary() {
+  const candidates = [
+    path.join(__dirname, '..', 'fetch'),
+    path.join(__dirname, '..', 'akai-fetch'),
+    path.join(process.resourcesPath, 'fetch'),
+    path.join(process.resourcesPath, 'akai-fetch'),
+  ];
+  for (const p of candidates) {
+    try {
+      require('fs').accessSync(p, require('fs').constants.X_OK);
+      return p;
+    } catch (_) {}
+  }
+  return null;
+}
+
+async function startServer() {
+  const bin = findServerBinary();
+  if (!bin) {
+    dialog.showErrorBox('Missing Binary',
+      'Could not find fetch/akai-fetch binary. Place it next to the app or in resources.');
+    app.quit();
+    return;
+  }
+
+  const usePort = process.env.AKAI_PORT ? parseInt(process.env.AKAI_PORT) : 0;
+
+  return new Promise((resolve, reject) => {
+    serverProcess = spawn(bin, ['serve', '-p', String(usePort), '--no-browser'], {
+      stdio: ['pipe', 'pipe', 'pipe'],
+    });
+
+    let output = '';
+    serverProcess.stdout.on('data', (data) => {
+      output += data.toString();
+      const m = output.match(/Local:\s+http:\/\/localhost:(\d+)/);
+      if (m) {
+        serverPort = parseInt(m[1]);
+        resolve();
+      }
+    });
+
+    serverProcess.stderr.on('data', (data) => {
+      console.error('[server]', data.toString().trim());
+    });
+
+    serverProcess.on('error', (err) => {
+      console.error('Failed to start server:', err);
+      reject(err);
+    });
+
+    serverProcess.on('close', (code) => {
+      if (serverPort === 0) {
+        reject(new Error(`Server exited with code ${code}`));
+      }
+    });
+  });
+}
+
+async function createWindow() {
+  try {
+    await startServer();
+  } catch (err) {
+    dialog.showErrorBox('Server Error', String(err));
+    app.quit();
+    return;
+  }
+
+  mainWindow = new BrowserWindow({
+    width: 1200,
+    height: 800,
+    minWidth: 800,
+    minHeight: 600,
+    title: 'AKAI Utils',
+    backgroundColor: '#1a1a2e',
+    webPreferences: {
+      preload: path.join(__dirname, 'preload.js'),
+      contextIsolation: true,
+      nodeIntegration: false,
+    },
+  });
+
+  mainWindow.loadURL(`http://localhost:${serverPort}`);
+  mainWindow.setMenuBarVisibility(false);
+
+  mainWindow.on('closed', () => {
+    mainWindow = null;
+  });
+}
+
+app.whenReady().then(createWindow);
+
+app.on('window-all-closed', () => {
+  if (serverProcess) {
+    serverProcess.kill();
+    serverProcess = null;
+  }
+  if (process.platform !== 'darwin') {
+    app.quit();
+  }
+});
+
+app.on('activate', () => {
+  if (mainWindow === null) {
+    createWindow();
+  }
+});
+
+app.on('before-quit', () => {
+  if (serverProcess) {
+    serverProcess.kill();
+    serverProcess = null;
+  }
+});

electron/package.json

@@ -0,0 +1,66 @@
+{
+  "name": "akai-utils",
+  "version": "0.2.0",
+  "description": "AKAI Sampler ISO Downloader, WAV Extractor & Library Manager",
+  "main": "main.js",
+  "build": {
+    "appId": "dev.akai.utils",
+    "productName": "AKAI Utils",
+    "directories": {
+      "output": "dist"
+    },
+    "files": [
+      "main.js",
+      "preload.js",
+      "package.json"
+    ],
+    "extraResources": [
+      {
+        "from": "../fetch",
+        "to": "fetch"
+      },
+      {
+        "from": "../scripts/extract_wavs.sh",
+        "to": "extract_wavs.sh"
+      }
+    ],
+    "mac": {
+      "category": "public.app-category.music",
+      "target": [
+        {
+          "target": "dir",
+          "arch": ["x64", "arm64"]
+        }
+      ],
+      "artifactName": "${name}-${version}-${arch}.${ext}",
+      "extraResources": [
+        {
+          "from": "../fetch",
+          "to": "fetch"
+        },
+        {
+          "from": "../scripts/extract_wavs.sh",
+          "to": "extract_wavs.sh"
+        },
+        {
+          "from": "../third_party/akaiutil/akaiutil",
+          "to": "akaiutil"
+        }
+      ]
+    },
+    "linux": {
+      "target": ["dir"],
+      "category": "Audio"
+    }
+  },
+  "scripts": {
+    "start": "electron .",
+    "build": "electron-builder",
+    "build:mac": "electron-builder --mac",
+    "build:linux": "electron-builder --linux"
+  },
+  "devDependencies": {
+    "electron": "^33.0.0",
+    "electron-builder": "^25.0.0"
+  }
+}

electron/preload.js

@@ -0,0 +1,5 @@
+const { contextBridge } = require('electron');
+
+contextBridge.exposeInMainWorld('electronAPI', {
+  platform: process.platform,
+});