fix(files): give the file preview a working detach button

The button next to the file preview's close icon was Copy Content, whose
overlapping-pages glyph reads as a pop-out control - and for a PDF or any
media/binary preview it was completely dead: those branches never fill
filePreviewContent, so the click hit an empty-content guard and did nothing,
with no feedback.

There is now a real detach button that opens the previewed file in a browser
tab (raw route for PDFs/images/media/text, the server-converted PDF preview
for docx/pptx), severs window.opener by hand so a blocked pop-up stays
detectable, closes the overlay on success (which also stops any playing
media), and disarms on close so it can never open a stale file. The copy
button now toasts 'Nothing to copy in this preview' instead of staying
silent.

Verified live with Playwright against an isolated instance: button visible
and armed on a PDF preview, file-raw answers 200, clicking opens the URL and
tears the overlay down, text previews keep a working copy buffer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/public/index.html

@@ -567,6 +567,7 @@
           <div class="file-preview-actions">
             <button class="btn-icon-sm file-preview-edit-btn" id="filePreviewEditBtn" onclick="app.enterFilePreviewEdit()" title="Edit file" aria-label="Edit file" hidden><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M17 3a2.85 2.83 0 1 1 4 4L7.5 20.5 2 22l1.5-5.5z"/></svg></button>
             <button class="btn-icon-sm" onclick="app.copyFilePreviewContent()" title="Copy content">&#x2398;</button>
+            <button class="btn-icon-sm" id="filePreviewDetachBtn" onclick="app.detachFilePreview()" title="Open in new tab" aria-label="Open in new tab" hidden><svg width="13" height="13" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/><polyline points="15 3 21 3 21 9"/><line x1="10" y1="14" x2="21" y2="3"/></svg></button>
             <button class="btn-icon-sm" onclick="app.closeFilePreview()" title="Close">&times;</button>
           </div>
         </div>

src/web/public/panels-ui.js

@@ -3313,6 +3313,11 @@ Object.assign(CodemanApp.prototype, {
     // Stop whatever the previous preview was playing. Overwriting innerHTML
     // only DETACHES a <video>/<audio>; a detached media element keeps playing.
     this._stopFilePreviewMedia();
+    // Disarm detach until this load has a URL of its own: an early error return
+    // must not leave the button opening the PREVIOUS file in a new tab.
+    this.filePreviewDetachUrl = '';
+    const detachBtn = this.$('filePreviewDetachBtn');
+    if (detachBtn) detachBtn.hidden = true;
 
     // Show overlay with loading state
     overlay.classList.add('visible');
@@ -3340,6 +3345,19 @@ Object.assign(CodemanApp.prototype, {
       return;
     }
 
+    // Every branch below renders from one of these routes, so the detach button
+    // can always offer the same bytes in a browser tab: docx/pptx through the
+    // server-converted PDF preview, everything else through the raw route.
+    // (html/htm arrive as a download there by design — file-raw serves them
+    // attachment-only so widening READ never widens RUN.)
+    const officeDoc = ext === 'docx' || ext === 'pptx';
+    this.filePreviewDetachUrl = attachmentId
+      ? `/api/sessions/${sessionId}/attachments/${encodeURIComponent(attachmentId)}/${officeDoc ? 'preview' : 'raw'}`
+      : officeDoc
+        ? `/api/sessions/${sessionId}/file-preview?path=${encodeURIComponent(filePath)}`
+        : `/api/sessions/${sessionId}/file-raw?path=${encodeURIComponent(filePath)}`;
+    if (detachBtn) detachBtn.hidden = false;
+
     // Registered attachment: render straight from its by-id routes — images and
     // PDFs inline, Office docs via the server-converted PDF preview, text fetched
     // raw. (Workspace-path previews fall through to the file-content endpoint.)
@@ -3489,6 +3507,29 @@ Object.assign(CodemanApp.prototype, {
     // audible and keeps streaming from the server. Closing has to stop it.
     this._stopFilePreviewMedia();
     this.filePreviewContent = '';
+    this.filePreviewDetachUrl = '';
+    const detachBtn = this.$('filePreviewDetachBtn');
+    if (detachBtn) detachBtn.hidden = true;
+  },
+
+  /**
+   * Open the previewed file in a browser tab and close the overlay.
+   *
+   * window.open is called WITHOUT the 'noopener' feature string: with it the
+   * call returns null even on success, which would make a blocked pop-up
+   * indistinguishable from a working one. The opener link is severed by hand
+   * instead, and a null return then reliably means the browser blocked it, in
+   * which case the overlay stays up so the user has not lost the file.
+   */
+  detachFilePreview() {
+    if (!this.filePreviewDetachUrl) return;
+    const win = window.open(this.filePreviewDetachUrl, '_blank');
+    if (!win) {
+      this.showToast('Pop-up blocked: allow pop-ups for this site to detach previews', 'error');
+      return;
+    }
+    win.opener = null;
+    this.closeFilePreview();
   },
 
   /**
@@ -4127,6 +4168,10 @@ Object.assign(CodemanApp.prototype, {
       }).catch(() => {
         this.showToast('Failed to copy', 'error');
       });
+    } else {
+      // Media/PDF/binary previews have no text buffer. Saying so beats the
+      // dead-button silence this used to be.
+      this.showToast('Nothing to copy in this preview', 'info');
     }
   },