fix(ext/popup): revoke object URLs in Document detail teardown

Two leaks from 705b171:
1. Lazy-load thumb for image-mime primary attachments created
   URL.createObjectURL but never revoked. Now tracked in a
   module-level registry, revoked on teardown.
2. 🔍 preview toggle's object URL same issue. Now tracked, revoked
   on teardown + on toggle-off (when user clicks the preview button
   to collapse).

Download button's URL (already self-cleaning via setTimeout) left
untracked — no change.

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

extension/src/popup/components/types/document.ts

@@ -17,8 +17,23 @@ let activeKeyHandler: ((e: KeyboardEvent) => void) | null = null;
 let activeFormEscHandler: ((e: KeyboardEvent) => void) | null = null;
 let sectionsExpanded = false;
 
+const detailObjectUrls = new Set<string>();
+
+function trackDetailUrl(url: string): string {
+  detailObjectUrls.add(url);
+  return url;
+}
+
+function revokeAllDetailUrls(): void {
+  for (const url of detailObjectUrls) {
+    URL.revokeObjectURL(url);
+  }
+  detailObjectUrls.clear();
+}
+
 export function teardown(): void {
   teardownAttachmentsDisclosure();
+  revokeAllDetailUrls();
   if (activeKeyHandler) {
     document.removeEventListener('keydown', activeKeyHandler);
     activeKeyHandler = null;
@@ -296,7 +311,7 @@ export async function renderDetail(app: HTMLElement, item: Item): Promise<void>
       if (!resp || !resp.ok) return;
       const data = resp.data as { bytes: ArrayBuffer };
       const blob = new Blob([data.bytes], { type: primaryRef.mime_type });
-      const url = URL.createObjectURL(blob);
+      const url = trackDetailUrl(URL.createObjectURL(blob));
       if (thumb) thumb.innerHTML = `<img src="${url}" alt="" />`;
     }).catch(() => {});
   }
@@ -316,12 +331,20 @@ export async function renderDetail(app: HTMLElement, item: Item): Promise<void>
   document.getElementById('doc-preview')?.addEventListener('click', async () => {
     const sigblock = document.getElementById('doc-sigblock')!;
     const existingPreview = sigblock.querySelector('.document-signature-block__preview');
-    if (existingPreview) { existingPreview.remove(); return; }
+    if (existingPreview) {
+      const img = existingPreview.querySelector('img');
+      if (img?.src) {
+        URL.revokeObjectURL(img.src);
+        detailObjectUrls.delete(img.src);
+      }
+      existingPreview.remove();
+      return;
+    }
     const resp = await sendMessage({ type: 'download_attachment', itemId: item.id, attachmentId: primaryRef.id });
     if (!resp || !resp.ok) return;
     const data = resp.data as { bytes: ArrayBuffer };
     const blob = new Blob([data.bytes], { type: primaryRef.mime_type });
-    const url = URL.createObjectURL(blob);
+    const url = trackDetailUrl(URL.createObjectURL(blob));
     const preview = document.createElement('div');
     preview.className = 'document-signature-block__preview';
     preview.innerHTML = `<img src="${url}" alt="" />`;