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/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"
+  }
+}