Add wireguard-ui-multi core: multi-server DB, WireGuard manager, REST API, UI, installers

Implements the from-scratch multi-server WireGuard management fork per
CLAUDE.md spec: sqlite schema (servers/peers/audit_log/users), Curve25519
key generation, per-interface config rendering + wg-quick/systemd control,
nftables hook scaffolding, session+CSRF-protected REST API with QR code
and config download endpoints, a minimal vanilla-JS web UI, legacy
wg0.conf migration, and both a native installer and a Proxmox LXC
provisioning script (with auto-detected latest Debian template).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/ui/static/login.js

@@ -0,0 +1,25 @@
+document.getElementById("login-form").addEventListener("submit", async function (e) {
+  e.preventDefault();
+  const errEl = document.getElementById("login-error");
+  errEl.textContent = "";
+
+  const form = e.target;
+  const username = form.username.value;
+  const password = form.password.value;
+
+  try {
+    const res = await fetch("/api/login", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ username, password }),
+    });
+    if (!res.ok) {
+      const data = await res.json().catch(() => ({}));
+      errEl.textContent = data.error || "Anmeldung fehlgeschlagen.";
+      return;
+    }
+    window.location.href = "/";
+  } catch (err) {
+    errEl.textContent = "Verbindung fehlgeschlagen.";
+  }
+});