fix(browse): trim markdown read-only banner + drop YAML front-matter placeholder

- Read-only markdown files mount as Toast UI Viewer, which already
  has no edit toolbar / no caret — the absence is itself the cue.
  Drop the explicit red banner; keep the disabled-Save tooltip.
- YAML front-matter textarea no longer shows a placeholder example
  (title/date/tags). A file without front matter renders as a
  genuinely empty pane instead of looking like it has content.
- YAML editor's banner stays — CodeMirror readOnly has no
  built-in visual signal beyond the disabled caret.

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

browse/css/base.css

@@ -41,11 +41,12 @@ body {
 .status-bar--error { color: #b00020; }
 .status-bar--info  { color: var(--primary); }
 
-/* Read-only banners — surfaced by preview-markdown.js / preview-yaml.js
-   when the listing's `writable` bit was false. Inline at the top of
-   the editor host so the user can't miss it; muted styling so it
-   doesn't fight the editor chrome. */
-.md-readonly-banner,
+/* Read-only banner for the YAML editor — surfaced by preview-yaml.js
+   when the listing's `writable` bit was false. CodeMirror's readOnly
+   mode has no built-in visual signal beyond the disabled caret, so a
+   banner here is the explicit cue. The markdown editor doesn't need
+   one because its read-only mount uses Toast UI's Viewer (no edit
+   toolbar at all). */
 .yaml-readonly-banner {
     background: rgba(220, 53, 69, 0.10);
     color: var(--text);

browse/js/preview-markdown.js

@@ -373,7 +373,10 @@
         fmTextarea.spellcheck = false;
         fmTextarea.autocapitalize = 'off';
         fmTextarea.autocomplete = 'off';
-        fmTextarea.placeholder = 'title: Document Title\ndate: 2026-05-13\ntags: [example]';
+        // No placeholder text — files with no YAML front matter render
+        // as a genuinely empty pane. Showing a synthetic example would
+        // make the file look like it had data when it doesn't.
+        fmTextarea.placeholder = '';
         fmBody.appendChild(fmTextarea);
         fmSection.appendChild(fmHeader);
         fmSection.appendChild(fmBody);
@@ -530,19 +533,13 @@
         var editor;
         if (!writableMode) {
             // Read-only mount uses Toast UI's Viewer (rendered markdown,
-            // no edit toolbar, no caret). Clear visual signal that the
-            // file isn't editable — much better than letting the user
-            // type into an Editor whose Save button is disabled.
+            // no edit toolbar, no caret). The disabled Save button +
+            // its tooltip carry the read-only signal — no banner here
+            // since the Viewer's lack of edit chrome is already a
+            // clear visual cue.
             editor = window.toastui.Editor.factory(Object.assign({}, editorOpts, {
                 viewer: true,
             }));
-            // Read-only banner above the editor explains why.
-            var roBanner = document.createElement('div');
-            roBanner.className = 'md-readonly-banner';
-            roBanner.innerHTML = '<span aria-hidden="true">🔒</span>'
-                + ' Read-only — you don\'t have write access to this file.'
-                + ' Save is disabled; changes won\'t persist.';
-            editorHost.insertBefore(roBanner, editorHost.firstChild);
         } else {
             editor = new window.toastui.Editor(Object.assign({}, editorOpts, {
                 // WYSIWYG by default — most users want the rendered view