fix(browse): empty rename-cue box; signal sync-on-open changes

The rename-cue div used an inline display:flex, which outranks the `hidden`
attribute's [hidden]{display:none} — so fmWarn.hidden=true never hid it and an
empty yellow box showed whenever the cue had nothing to say. Control visibility
via style.display ('none'/'flex') instead of the hidden attribute.

Also surface a status line when sync-on-open rewrites the front matter to match
the filename, so the change isn't silent ("Front matter synced to filename —
review and save").

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

browse/js/preview-markdown.js

@@ -450,11 +450,14 @@
         // discarding the value. Populated by renderIdentityCue().
         var fmWarn = document.createElement('div');
         fmWarn.className = 'md-fm__warn';
-        fmWarn.hidden = true;
+        // Visibility is controlled via style.display (toggled in
+        // renderIdentityCue), NOT the `hidden` attribute: an inline
+        // display:flex outranks [hidden]{display:none}, which would leave an
+        // empty box on screen whenever the cue has nothing to say.
         fmWarn.style.cssText = 'color:#92400e;background:#fffbeb;border:1px solid '
             + '#fcd34d;border-radius:4px;padding:6px 8px;margin:0 0 4px;font-size:'
-            + '0.78rem;line-height:1.5;display:flex;flex-wrap:wrap;align-items:'
+            + '0.78rem;line-height:1.5;flex-wrap:wrap;align-items:center;gap:6px;'
-            + 'center;gap:6px;';
+            + 'display:none;';
         fmSection.appendChild(fmHeader);
         fmSection.appendChild(fmWarn);
         fmSection.appendChild(fmBody);
@@ -822,7 +825,7 @@
         function renderIdentityCue() {
             while (fmWarn.firstChild) fmWarn.removeChild(fmWarn.firstChild);
             var fid = filenameIdentity(node.name);
-            if (!fid || !canSave(node)) { fmWarn.hidden = true; return; }
+            if (!fid || !canSave(node)) { fmWarn.style.display = 'none'; return; }
             var data = parseFrontMatter('---\n' + fmTextarea.value + '\n---\n').data || {};
             var edits = [];
             IDENTITY_FIELDS.forEach(function (f) {
@@ -831,7 +834,7 @@
                 var want = String(fid[f.fm] == null ? '' : fid[f.fm]).trim();
                 if (got !== '' && got !== want) edits.push(f.label + ' → “' + got + '”');
             });
-            if (!edits.length) { fmWarn.hidden = true; return; }
+            if (!edits.length) { fmWarn.style.display = 'none'; return; }
             var msg = document.createElement('span');
             msg.textContent = '✎ Identity comes from the filename. You changed '
                 + edits.join(', ') + '. ';
@@ -846,7 +849,7 @@
                 btn.addEventListener('click', function () { renameToMatch(newName); });
                 fmWarn.appendChild(btn);
             }
-            fmWarn.hidden = false;
+            fmWarn.style.display = 'flex';
         }
 
         // Rename action: persist the current buffer (so body edits aren't
@@ -911,8 +914,12 @@
         renderIdentityCue(); // initial state on load (clean after sync-on-open)
 
         // If sync-on-open corrected the front matter, open the buffer dirty so
-        // a save bakes the filename-derived identity in.
+        // a save bakes the filename-derived identity in — and say so, since the
-        if (writableMode && fmTextarea.value !== onDiskFM) markDirty(true);
+        // change is otherwise silent (the values just match the filename now).
+        if (writableMode && fmTextarea.value !== onDiskFM) {
+            markDirty(true);
+            statusEl.textContent = 'Front matter synced to filename — review and save';
+        }
 
         // ── Save ───────────────────────────────────────────────────────────
         // Mark a successful write: adopt the new server ETag (so the next