attachments preview overlay, docx/xlsx/odf rendering, composer drag-drop & undo-send UX

Frontend:
- Full-screen attachment preview overlay (AttachmentPreview) for image/pdf/txt
  plus office docs: .docx via docx-preview (rendered live into the sandboxed
  iframe so header/footer VML images survive), .xlsx via xlsx-preview, and
  .odt/.ods/.odp/.odg via a JSZip+DOMParser ODF walker (officeConvert).
- Type-aware attachment icons (green spreadsheet, blue document, red PDF,
  image/archive); new document/image/spreadsheet/print SVGs.
- Composer: drag-drop attachments (body drop inserts images inline, else
  attaches); inserted images are selectable/resizable/deletable (RichTextEditor).
- Undo-send: "Sending…" banner with live "N seconds to Undo" countdown.
- Search result list: pin/archive/delete update correctly; grey-out only for
  search hits; scoped "Search in Trash/Spam".

Backend:
- Search: prefix (search-as-you-type) matching, email-address search, and
  address-first result ordering; Trash/Junk excluded with scoped folder search.
- Attachments: RFC 6266 Content-Disposition for non-ASCII filenames; one-time
  inline image loading.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rcj3TpydMTzTSmU7kunzf8

frontend/package.json

@@ -15,10 +15,14 @@
   "dependencies": {
     "@tanstack/react-query": "~5.51.0",
     "@tanstack/react-virtual": "~3.8.0",
+    "docx-preview": "^0.4.0",
+    "jszip": "^3.10.1",
+    "pdfjs-dist": "^4.10.38",
     "qrcode.react": "~4.1.0",
     "react": "~18.3.1",
     "react-dom": "~18.3.1",
     "react-router-dom": "~6.26.0",
+    "xlsx-preview": "^1.0.5",
     "zod": "~3.23.8"
   },
   "devDependencies": {