feat(form): friendly empty-state when standalone HTML opened directly

The form renderer is server-driven: zddc-server's form handler injects
schema/ui/data into <script id="form-context"> on every render.
Standalone form/dist/form.html opened from file:// has no context and
just rendered as a blank page below the header chrome — no orientation,
no submit button affordance was hidden, but nothing labelled what the
user was looking at.

Detect the empty-context case in form/js/main.js, render a small
.form-welcome card explaining that this tool is server-driven, and hide
the now-meaningless Submit button. The card uses shared tokens
(--bg, --border, --font-mono, etc.) so it themes correctly.

Note: the unified tables.html bundle that hosts the form via the
zddcMode dispatcher is unaffected — tables always sets zddcMode='form'
or 'table' before form's main.js boots, so the welcome only fires for
the standalone HTML.

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

form/css/form.css

@@ -198,3 +198,31 @@
     opacity: 0.5;
     cursor: not-allowed;
 }
+
+/* Standalone welcome — shown when form.html is opened directly (no server-injected #form-context). */
+.form-welcome {
+    max-width: 36rem;
+    margin: 2rem auto;
+    padding: 1.5rem 1.75rem;
+    background: var(--bg);
+    border: 1px solid var(--border);
+    border-radius: var(--radius);
+}
+.form-welcome h2 {
+    margin-bottom: 0.5rem;
+    font-size: 1.25rem;
+}
+.form-welcome h3 {
+    margin: 1rem 0 0.35rem;
+    font-size: 0.95rem;
+}
+.form-welcome p { margin-bottom: 0.75rem; line-height: 1.5; }
+.form-welcome ol { margin: 0 0 0.75rem 1.25rem; }
+.form-welcome li { margin-bottom: 0.35rem; }
+.form-welcome code {
+    font-family: var(--font-mono);
+    font-size: 0.85em;
+    background: var(--bg-secondary);
+    padding: 0.05em 0.3em;
+    border-radius: 3px;
+}

form/js/main.js

@@ -1,6 +1,37 @@
 (function (app) {
     'use strict';
 
+    // Friendly empty-state shown when the form is opened standalone
+    // (file:// or otherwise without a server-injected #form-context
+    // payload). The form renderer is always driven by the host —
+    // zddc-server's form handler injects schema+ui+data; the tool has
+    // no client-side picker because there's nothing it could pick from
+    // outside that contract.
+    function renderStandaloneWelcome(root) {
+        if (!root) return;
+        root.innerHTML = '';
+        const wrap = document.createElement('div');
+        wrap.className = 'form-welcome';
+        wrap.innerHTML = [
+            '<h2>ZDDC Form Renderer</h2>',
+            '<p>This tool renders a form spec injected by <code>zddc-server</code>',
+            ' at <code>&lt;name&gt;.form.html</code> URLs. There is no schema',
+            ' to render here — most likely you opened the standalone HTML directly.</p>',
+            '<h3>To use it</h3>',
+            '<ol>',
+            '<li>Run <code>zddc-server</code> against an archive that contains a',
+            ' <code>&lt;name&gt;.form.yaml</code> spec.</li>',
+            '<li>Visit <code>&lt;path&gt;/&lt;name&gt;.form.html</code> in the browser.</li>',
+            '</ol>',
+            '<p>See <a href="https://zddc.varasys.io/reference.html" target="_blank" rel="noopener">',
+            'zddc.varasys.io/reference.html</a> for the full ZDDC reference.</p>'
+        ].join('');
+        root.appendChild(wrap);
+
+        const submitBtn = document.getElementById('submit-btn');
+        if (submitBtn) submitBtn.hidden = true;
+    }
+
     function boot() {
         // When this bundle is hosted by the unified tables.html, the
         // mode dispatcher decides which app paints. Skip when mode is
@@ -32,6 +63,12 @@
                 app.context.ui || {},
                 app.context.data
             );
+        } else if (root) {
+            // No schema — server-injected context is empty. Most common
+            // when the standalone form.html is opened from file:// without
+            // a host. Show a friendly explanation instead of a blank page.
+            renderStandaloneWelcome(root);
+            return;
         }
 
         if (app.context.errors && app.context.errors.length) {