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/src/types/xlsx-preview.d.ts

@@ -0,0 +1,11 @@
+// xlsx-preview ships types in dist/index.d.ts but doesn't declare them in
+// package.json, so TS can't find them. Minimal ambient declaration for the API
+// we use: xlsx2Html(ArrayBuffer|Blob|File) -> Promise<HTML string>.
+declare module "xlsx-preview" {
+  export function xlsx2Html(
+    data: ArrayBuffer | Blob | File,
+    options?: Record<string, unknown>,
+  ): Promise<string | ArrayBuffer | string[]>;
+  const _default: { xlsx2Html: typeof xlsx2Html };
+  export default _default;
+}