Add welcome surface and user default surface preference

- Create welcome surface: topbar + getting-started card shown when
  no extension surfaces are installed. Admins see a link to Packages,
  non-admins see a message to contact their administrator.
- Update resolveDefaultSurface priority chain:
  user preference → global config → first extension → /welcome
- Register welcome as a core surface with authenticated auth
- Final fallback is now /welcome instead of /admin, breaking the
  infinite back-button loop

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/js/sw/surfaces/welcome/index.js

@@ -0,0 +1,43 @@
+/**
+ * WelcomeSurface — shown when no extension surfaces are installed.
+ * Renders Topbar + a getting-started card with link to admin.
+ */
+const { html } = window;
+const { render } = preact;
+
+import { Topbar } from '../../shell/topbar.js';
+
+function WelcomeSurface() {
+    const BASE = window.__BASE__ || '';
+    const isAdmin = sw.can?.('surface.admin.access');
+
+    return html`
+        <${Topbar} title="Switchboard" />
+        <div style="flex:1;display:flex;align-items:center;justify-content:center;padding:24px;">
+            <div style="max-width:420px;width:100%;background:var(--bg-surface);border:1px solid var(--border);border-radius:12px;padding:32px;text-align:center;">
+                <div style="font-size:36px;margin-bottom:16px;">📦</div>
+                <h2 style="font-size:20px;font-weight:600;margin:0 0 8px;">Welcome to Switchboard</h2>
+                <p style="font-size:14px;color:var(--text-2);margin:0 0 24px;line-height:1.5;">
+                    No surfaces are installed yet. Install an extension surface
+                    to get started.
+                </p>
+                ${isAdmin && html`
+                    <a href="${BASE}/admin/packages"
+                       style="display:inline-flex;align-items:center;gap:8px;padding:10px 24px;background:var(--accent);color:#fff;border-radius:8px;text-decoration:none;font-weight:600;font-size:14px;">
+                        Go to Packages
+                    </a>
+                `}
+                ${!isAdmin && html`
+                    <p style="font-size:13px;color:var(--text-3);">
+                        Ask your administrator to install a surface package.
+                    </p>
+                `}
+            </div>
+        </div>
+    `;
+}
+
+const mount = document.getElementById('welcome-mount');
+if (mount) {
+    render(html`<${WelcomeSurface} />`, mount);
+}