ui: replace HTML textarea with drag-and-drop file zone

Publish form now uses a single dashed drop area for .html uploads
(click-to-browse fallback included). Removes the paste-HTML textarea
since real artifacts always come from a file. Server-side multipart
handling is unchanged; the `file` form field already took precedence.

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

internal/server/templates/dashboard.html

@@ -26,9 +26,21 @@
   <!-- Publish form -->
   <section class="rounded border border-slate-800 bg-slate-900/50 p-4 mb-8">
     <h2 class="text-lg font-semibold mb-3">Publish</h2>
-    <form method="post" action="/api/publish" enctype="multipart/form-data" class="space-y-3">
+    <form method="post" action="/api/publish" enctype="multipart/form-data" class="space-y-3" id="publishForm">
-      <textarea name="html" rows="8" placeholder="Paste HTML here, or use the file picker below"
+      <label for="dropInput"
-                class="w-full bg-slate-950 border border-slate-800 rounded px-3 py-2 text-sm focus:outline-none focus:border-indigo-500"></textarea>
+             id="dropZone"
+             class="flex flex-col items-center justify-center text-center cursor-pointer
+                    border-2 border-dashed border-slate-700 hover:border-indigo-500
+                    rounded-lg bg-slate-950/50 hover:bg-slate-950 transition-colors
+                    px-6 py-12 text-sm text-slate-400">
+        <svg class="w-10 h-10 mb-3 text-slate-500" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5"
+                d="M7 16a4 4 0 01-.88-7.9A5 5 0 0115.9 6.56 4 4 0 1118 16M8 13l4-4m0 0l4 4m-4-4v10"/>
+        </svg>
+        <div><span class="text-indigo-300 font-medium">Drop HTML file here</span> or click to browse</div>
+        <div class="text-xs text-slate-500 mt-1" id="dropHint">.html · up to 5 MB</div>
+        <input id="dropInput" type="file" name="file" accept=".html,text/html" class="sr-only" required>
+      </label>
       <div class="grid grid-cols-1 md:grid-cols-4 gap-3">
         <label class="block">
           <span class="block text-xs text-slate-400 mb-1">Custom slug (optional)</span>
@@ -56,15 +68,55 @@
           </select>
         </label>
       </div>
-      <div class="flex items-center justify-between">
+      <div class="flex items-center justify-end">
-        <label class="text-xs text-slate-400">
-          Or upload: <input type="file" name="file" accept=".html,text/html" class="ml-2 text-xs">
-        </label>
         <button class="bg-indigo-600 hover:bg-indigo-500 text-white rounded px-4 py-2 font-medium text-sm">Publish</button>
       </div>
     </form>
   </section>
 
+  <script>
+    (function () {
+      const zone = document.getElementById('dropZone');
+      const input = document.getElementById('dropInput');
+      const hint = document.getElementById('dropHint');
+      const form = document.getElementById('publishForm');
+      if (!zone || !input) return;
+
+      const ACTIVE = ['border-indigo-500', 'bg-indigo-500/10'];
+      const prevent = e => { e.preventDefault(); e.stopPropagation(); };
+      ['dragenter', 'dragover'].forEach(ev =>
+        zone.addEventListener(ev, e => { prevent(e); zone.classList.add(...ACTIVE); }));
+      ['dragleave', 'drop'].forEach(ev =>
+        zone.addEventListener(ev, e => { prevent(e); zone.classList.remove(...ACTIVE); }));
+
+      zone.addEventListener('drop', e => {
+        const f = e.dataTransfer && e.dataTransfer.files && e.dataTransfer.files[0];
+        if (!f) return;
+        if (!/\.html?$/i.test(f.name) && f.type !== 'text/html') {
+          hint.textContent = 'Only .html files are accepted';
+          hint.classList.add('text-red-400');
+          return;
+        }
+        // DataTransfer into the input so form submit picks it up.
+        const dt = new DataTransfer();
+        dt.items.add(f);
+        input.files = dt.files;
+        showSelection(f);
+      });
+
+      input.addEventListener('change', () => {
+        if (input.files && input.files[0]) showSelection(input.files[0]);
+      });
+
+      function showSelection(f) {
+        const kb = (f.size / 1024).toFixed(1);
+        hint.textContent = `${f.name} · ${kb} KB`;
+        hint.classList.remove('text-red-400');
+        hint.classList.add('text-indigo-300');
+      }
+    })();
+  </script>
+
   {{ if .FlashSlug }}
   <div class="rounded border border-emerald-800 bg-emerald-900/30 text-emerald-100 px-4 py-3 mb-6 text-sm flex items-center justify-between">
     <div>