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

main.go

@@ -68,6 +68,8 @@ func main() {
 		cmdExtract(args)
 	case "pipeline":
 		cmdPipeline(args)
+	case "serve":
+		cmdServe(args)
 	default:
 		fmt.Fprintf(os.Stderr, "unknown command: %s\n", cmd)
 		printUsage()
@@ -84,6 +86,7 @@ Usage:
   akai-fetch download [flags]           download ISOs (from file or by identifier)
   akai-fetch extract [flags]            extract WAVs from downloaded ISOs
   akai-fetch pipeline [flags]           full search → download → extract pipeline
+  akai-fetch serve [flags]              start HTTP server with web UI
 
 Search flags:
   -query "akai sampler"    search query (default: "subject:akai AND subject:sampler")
@@ -109,7 +112,11 @@ Pipeline flags:
   -limit 30                max results
   -dir "./output"          base output directory
   -download-jobs 2         concurrent downloads
-  -extract-jobs 2          concurrent extractions`)
+  -extract-jobs 2          concurrent extractions
+
+Serve flags:
+  -p, --port 0             HTTP port (0 = auto-assign)
+  --no-browser             don't open browser automatically`)
 }
 
 // ─── Search ────────────────────────────────────────────────────────────