Add standalone Go build alongside VSCode extension

Ports all five tools to a single 7.5 MB Windows exe with an embedded
WebView2 window, file-based config, single-instance focus, and clean
shutdown. Adds build.ps1 to build both targets from one command.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

standalone/home.go

@@ -0,0 +1,19 @@
+package main
+
+import "net/http"
+
+func HomePage(w http.ResponseWriter, r *http.Request) {
+	body := `
+<h2>HPL Toolbox</h2>
+<p class="hint">Standalone build. Pick a tool from the top bar.</p>
+<ul style="line-height:1.9;">
+  <li><a href="/plec" style="color:#9cdcfe;">PLEC Upload</a> — Upload HTML to the internal PLEC server</li>
+  <li><a href="/applovin" style="color:#9cdcfe;">AppLovin Demo Upload</a> — Upload to p.applov.in (QR preview)</li>
+  <li><a href="/base64" style="color:#9cdcfe;">Base64 Scanner</a> — Find non-base64 assets in HTML</li>
+  <li><a href="/daily" style="color:#9cdcfe;">Daily Update</a> — Compose &amp; copy a daily status</li>
+  <li><a href="/mobile" style="color:#9cdcfe;">Send To Mobile</a> — Share HTML to a phone via LAN + QR</li>
+</ul>
+`
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_, _ = w.Write([]byte(Page("/", "Home", body)))
+}