desktop: Import and Export work offline; the menu toggle is reachable; errors say why

Export and Import were a link to the server and a reject("needs a server") on the
desktop. Both now run in the core with no server:

- core/src/local/portable.rs builds the same zip the server writes (notes.json,
  a Markdown file per note, each attachment this device holds) and reads either
  export marker or a Google Keep Takeout zip, with the server's decompression
  budget and an all-or-nothing transaction. Export saves to Downloads (no new
  plugin) and the sidebar says where; Import takes the archive as raw IPC bytes.
- core/testdata/portable.json pins the format for both copies: the server runs
  its Keep and native readers against it (test_portable_fixture.py) and checks
  its real export's keys (test_integration.py); the core runs the same cases.
- Found on the way: both importers skipped a Keep note that is only a photo as
  "empty". It now imports, on the server and in the core.
- New dependency, approved: `zip` (deflate only) plus `flate2` on its pure-Rust
  backend, both already in the lockfile.

The AppImage applications-menu toggle moves from Account, which the desktop
never shows, to the Sync page; the first-run prompt now says so.

errorMessage (#5236) replaces the hand-rolled `.error ?? …` / `.message ?? e`
reads at the remaining catch sites, so a desktop failure shows its real reason.

Task #5170.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/adapters/local.ts

@@ -4,9 +4,9 @@
 //
 // Argument keys are camelCase; Tauri converts them to the Rust commands' snake_case
 // parameters (e.g. labelIds -> label_ids). A few operations have no offline meaning
-// yet (account auth, device linking, URL unfurl, file import) — those reject with a
-// clear message rather than silently failing; the board, editor, attachments,
-// capture, filters, labels, checklists and reminders all work fully offline.
+// yet (account auth, device linking, URL unfurl) — those reject with a clear message
+// rather than silently failing; the board, editor, attachments, capture, filters,
+// labels, checklists, reminders, import and export all work fully offline.
 
 import { invoke } from "../desktop/bridge";
 import type { Note, NoteRevision } from "../stores/notes";
@@ -72,7 +72,9 @@ export const local: Repo = {
     deleteAttachment: (id, attId) => invoke<Note>("notes_delete_attachment", { id, attId }),
     unfurl: () => Promise.reject<Note>(new Error(NEEDS_SERVER)),
     deletePreview: (id, previewId) => invoke<Note>("notes_delete_preview", { id, previewId }),
-    import: () => Promise.reject<ImportResult>(new Error(NEEDS_SERVER)),
+    // The archive crosses as raw bytes, as an attachment does.
+    import: async (file) => invoke<ImportResult>("notes_import", new Uint8Array(await file.arrayBuffer())),
+    exportAll: () => invoke<string>("notes_export"),
     reorder: (orderedIds) => invoke<void>("notes_reorder", { orderedIds }),
     trash: (id) => invoke<Note>("notes_trash", { id }),
     restore: (id) => invoke<Note>("notes_restore", { id }),