Document Upload Editing Safeguards, Bulk Document Download, and Grid/UI Polish (QA)

This QA batch bundles several frontend improvements centered on the document-upload workflow and mail document handling. Users editing a PDF's document segments can now cancel and roll back their changes, document-type dropdowns load faster with searchable pagination, mail details gains a one-click bulk download for all documents, and long text across batch grids is consistently truncated with hover tooltips. A branded 404 page and an Add Mail form fix (a selectable "None" option for Client/Subclient) are also included.

Several rough edges had accumulated in the document and batch experiences. While splitting a PDF into document segments, users had no safe way to abandon in-progress edits, risking accidental changes. Document-type dropdowns fetched up to 50 items at once, slowing the first open. Downloading all documents on a mail required repetitive per-file actions. Long values in batch grids overflowed without a way to read the full text, and the Add Mail form offered no way to clear an already-chosen Client or Subclient. There was also no branded experience for unknown URLs.

• Operators editing document segments can press "Cancel Changes" to discard interim edits and restore the prior state, reducing data-entry mistakes.
• Document-type pickers open faster (smaller initial page) and support type-ahead search plus load-more for large catalogs.
• Mail Details adds a bulk "Download" action that packages all of a mail's documents in one request.
• Batch grid cells truncate long strings with a tooltip showing the full value, improving readability.
• Add Mail lets users explicitly select "None" to clear Client/Subclient.
• Visiting an unknown URL now shows a branded 404 page with a "Return to inbox" link.

Grounded in the diff (~724 add / 134 del across 19 files), bundling feature PRs #658, #659, #661, #662, #663:

• Document upload (document-upload/): New createSnapshot/restoreSnapshot in useDocumentSegmentsStore with deep-cloned segments; page captures pre-edit and committed snapshots so Cancel rolls back. Button relabeled "Cancel Changes".
• Document-type options (useDocumentTypeOptions): Initial page size cut from 50 to 10; added a cached, de-duplicated searchDocumentTypes(query,page,limit) async loader wired into MultipleSelector via onPaginatedSearch.
• Segments grid: Skeleton ScrollSeekPlaceholder and tuned virtualization padding for fast scrolls; "Add Range" hidden while segments/PDF context are still loading; gated on onLoadSuccess/onLoadError.
• Bulk download: New route /v1/downloads/documents/by-mail, service bulkDownloadDocumentsByMail, and useDownloadMailDocumentsMutation; wired into MailDocuments download button.
• MailDocuments: Reworked "uncategorized" bucket to filter type-less docs client-side; dropped unassignedCategoryId lookup and include_columns/include_version_info params.
• Batches grid: MAX_TRUNCATED_CHARS_TO_SHOW=50 constant; TruncatedText applied to string cells in data-grid.tsx and BatchContents/columns.tsx; tightened column typing.
• MailForm: "None" SelectItem clears Client/Subclient.
• 404: New not-found.tsx + branded NotFoundContent.
• Batches documents page: Removed useGetDocumentStatus/Completed-status fallback; documents fetch only when a status is selected.
• Note: PR description also lists adminhub/Strapi fixes, but no such files appear in this diff.

Medium. The snapshot save/cancel logic in document upload has many new effects and refs, so the main risk is stale or incorrectly restored state on cancel/save. The shared paginated-search cache could surface stale or mismatched dropdown pages if cache keys collide. The grid truncation change clones React elements and replaces string children, which could mis-render cells whose content is not a simple string. The batches-documents page now fetches only when a status is selected, changing default load behavior. Mitigation: deep clones isolate snapshot state, the cache is keyed by query+limit+page, and element cloning falls through to render-as-is for non-string children; QA should exercise these paths directly.

• Document upload: enter edit mode, modify segments/ranges/notes, click "Cancel Changes" and confirm full rollback; save and confirm baseline updates; verify "Add Range" stays hidden until PDF/segments finish loading.
• Document-type dropdowns: confirm fast initial open, type-ahead search returns server results, and load-more works for large catalogs without duplicates.
• Mail Details: click bulk "Download", confirm all documents are packaged; verify error toast when mail id is missing.
• Uncategorized bucket: confirm only type-less documents appear and counts are correct.
• Batch grids: confirm long strings truncate with a working tooltip and short values render normally.
• Add Mail: select "None" for Client and Subclient and confirm selection clears.
• Navigate to an invalid URL and confirm the branded 404 with a working "Return to inbox" link.
• Batches > Documents tab: confirm documents load when a status is selected and the tab still behaves correctly with no status.