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/components/AppShell.vue

@@ -10,6 +10,7 @@ import { useUiStore } from "../stores/ui";
 import BaseModal from "./BaseModal.vue";
 import CommandPalette from "./CommandPalette.vue";
 import Icon from "./Icon.vue";
+import ExportNotes from "./ExportNotes.vue";
 import ImportNotes from "./ImportNotes.vue";
 import LabelsModal from "./LabelsModal.vue";
 import { isDesktop } from "../desktop/bridge";
@@ -484,10 +485,7 @@ async function signOut() {
           >
             <Icon name="calendar" /> Timeline
           </RouterLink>
-          <!-- Direct download (same-origin GET, session cookie sent); not a route. -->
-          <a href="/api/notes/export" download class="nav-link" title="Download all your notes as a zip">
-            <Icon name="download" /> Export
-          </a>
+          <ExportNotes />
           <ImportNotes />
 
           <!-- Account, settings and sign-out, for the screens where they are NOT in