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/icons/image.svg

@@ -0,0 +1,35 @@
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+   width="26"
+   height="26"
+   viewBox="0 0 24 24"
+   fill="none"
+   version="1.1"
+   id="svg3"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+  <defs
+     id="defs3" />
+  <path
+     d="M6 2.5h7L19 8v13a1.5 1.5 0 0 1-1.5 1.5h-11A1.5 1.5 0 0 1 5 21V4A1.5 1.5 0 0 1 6 2.5z"
+     fill="#2563eb"
+     opacity="0.15"
+     id="path1"
+     style="fill:#eb9425;fill-opacity:1" />
+  <path
+     d="M13 2.5L19 8h-4.5A1.5 1.5 0 0 1 13 6.5V2.5z"
+     fill="#2563eb"
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+     id="path2"
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+  <path
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+     id="path3" />
+  <circle
+     style="fill:none;fill-opacity:1;stroke:#ff6600;stroke-width:0.719258;stroke-linecap:round;stroke-dasharray:none"
+     id="path4"
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+</svg>