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/views/SyncView.vue

@@ -4,6 +4,7 @@ import { useUiStore } from "../stores/ui";
 import BaseButton from "../components/BaseButton.vue";
 import BaseInput from "../components/BaseInput.vue";
 import Icon from "../components/Icon.vue";
+import DesktopMenuEntry from "../components/DesktopMenuEntry.vue";
 import {
   SUGGESTED_CAPTURE_SHORTCUT,
   capture as captureBridge,
@@ -20,6 +21,7 @@ import {
   type UpdateChannel,
   type UpdateStatus,
 } from "../desktop/bridge";
+import { errorMessage } from "../api/errors";
 
 // Opt-in server sync for the desktop app. Being UNLINKED is the normal resting
 // state, not an incomplete setup — the app is local-first and fully usable having
@@ -111,7 +113,7 @@ async function saveShortcut(value: string) {
     // The message comes from the core and names the actual reason — "something
     // else is already using it" reads very differently from "that is not a
     // shortcut this system understands", and both are things you can act on.
-    shortcutError.value = String((e as { message?: string }).message ?? e);
+    shortcutError.value = errorMessage(e, "Couldn't save the shortcut.");
   } finally {
     savingShortcut.value = false;
   }
@@ -124,7 +126,7 @@ async function checkUpdates() {
     update.value = await updateBridge.check();
     channel.value = update.value.channel;
   } catch (e) {
-    updateError.value = String((e as Error)?.message ?? e);
+    updateError.value = errorMessage(e, "The update check failed.");
   } finally {
     checking.value = false;
     checkedOnce.value = true;
@@ -141,7 +143,7 @@ async function switchChannel(next: UpdateChannel) {
     checkedOnce.value = false;
     await checkUpdates();
   } catch (e) {
-    updateError.value = String((e as Error)?.message ?? e);
+    updateError.value = errorMessage(e, "The update check failed.");
   }
 }
 
@@ -153,7 +155,7 @@ async function installUpdate() {
     // means it failed.
     await updateBridge.install();
   } catch (e) {
-    updateError.value = String((e as Error)?.message ?? e);
+    updateError.value = errorMessage(e, "The update check failed.");
   } finally {
     installing.value = false;
   }
@@ -191,7 +193,7 @@ async function runProbe() {
   try {
     probe.value = await syncBridge.probe(url.value);
   } catch (e) {
-    probeError.value = String((e as Error)?.message ?? e);
+    probeError.value = errorMessage(e, "Couldn't reach that server.");
   } finally {
     probing.value = false;
   }
@@ -219,7 +221,7 @@ async function connect() {
     ui.showToast(`Connected to ${result.status.server_url}.`);
     await syncNow();
   } catch (e) {
-    linkError.value = String((e as Error)?.message ?? e);
+    linkError.value = errorMessage(e, "Couldn't link to that server.");
   } finally {
     linking.value = false;
   }
@@ -261,7 +263,7 @@ async function syncNow() {
     showOutcome(await syncBridge.now());
     pending.value = await syncBridge.hasPending();
   } catch (e) {
-    syncError.value = String((e as Error)?.message ?? e);
+    syncError.value = errorMessage(e, "Sync failed.");
   } finally {
     syncing.value = false;
   }
@@ -303,7 +305,7 @@ async function disconnect() {
         : "Disconnected. This device now works offline only.",
     );
   } catch (e) {
-    ui.showToast(String((e as Error)?.message ?? e));
+    ui.showToast(errorMessage(e, "Couldn't disconnect."));
   }
 }
 
@@ -676,5 +678,9 @@ onBeforeUnmount(() => stopSyncListener?.());
         </BaseButton>
       </div>
     </section>
+
+    <!-- The AppImage's menu entry, which the first-run prompt says can be undone
+         from here. Hides itself on every other build. -->
+    <DesktopMenuEntry />
   </div>
 </template>