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>

core/testdata/portable.json

@@ -0,0 +1,146 @@
+{
+  "_about": [
+    "The export/import format, pinned for every implementation of it.",
+    "Read by the server (tests/test_portable_fixture.py, and the export check in test_integration.py) and by the core (local/portable.rs). The desktop's offline Export and Import are the core's copy; a backup taken on one surface has to restore on the other.",
+    "`keep` and `native` are notes as they arrive, with the spec each importer must make of them. `export` names the keys an export writes, at the top of notes.json and on each note. A difference between the server and the core here is a backup that loses something on the way across."
+  ],
+  "keep": [
+    {
+      "about": "A list note keeps its text as well as its items; annotation URLs are folded into the body; the colour is read past.",
+      "dir": "Takeout/Keep",
+      "note": {
+        "title": "Groceries",
+        "textContent": "for the weekend",
+        "listContent": [
+          { "text": "Milk", "isChecked": false },
+          { "text": "Eggs", "isChecked": true }
+        ],
+        "annotations": [{ "url": "https://example.com/recipe" }],
+        "labels": [{ "name": "shopping" }],
+        "color": "TEAL",
+        "isPinned": true,
+        "isArchived": false,
+        "isTrashed": false,
+        "createdTimestampUsec": 1600000000000000,
+        "userEditedTimestampUsec": 1600000100000000,
+        "attachments": [
+          { "filePath": "photo.jpg", "mimetype": "image/jpeg" },
+          { "filePath": "scan.png" }
+        ]
+      },
+      "spec": {
+        "title": "Groceries",
+        "body": "for the weekend\n\nhttps://example.com/recipe",
+        "pinned": true,
+        "archived": false,
+        "trashed": false,
+        "labels": ["shopping"],
+        "items": [
+          { "text": "Milk", "checked": false },
+          { "text": "Eggs", "checked": true }
+        ],
+        "attachments": [
+          { "file": "Takeout/Keep/photo.jpg", "mime": "image/jpeg" },
+          { "file": "Takeout/Keep/scan.png", "mime": "image/png" }
+        ],
+        "created_at": "2020-09-13T12:26:40Z",
+        "updated_at": "2020-09-13T12:28:20Z"
+      }
+    },
+    {
+      "about": "A trashed, archived text note with no timestamps.",
+      "dir": "",
+      "note": {
+        "textContent": "old idea",
+        "isArchived": true,
+        "isTrashed": true
+      },
+      "spec": {
+        "title": null,
+        "body": "old idea",
+        "pinned": false,
+        "archived": true,
+        "trashed": true,
+        "labels": [],
+        "items": [],
+        "attachments": [],
+        "created_at": null,
+        "updated_at": null
+      }
+    }
+  ],
+  "native": [
+    {
+      "about": "A current export: the checklist is already in the body, a reminder and recurrence come across, and the pre-M315 colour is read past.",
+      "note": {
+        "id": "6ba7b810-9dad-11d1-80b4-00c04fd430c8",
+        "display_title": "Trip",
+        "body": "Trip\n\n- [ ] passport",
+        "color": "blue",
+        "pinned": true,
+        "archived": false,
+        "remind_at": "2026-11-01T09:00:00+00:00",
+        "recurrence": "weekly",
+        "created_at": "2026-07-19T00:00:00+00:00",
+        "updated_at": "2026-07-20T08:30:00.250000+00:00",
+        "labels": ["travel"],
+        "attachments": [{ "file": "attachments/6ba7b810/0/ticket.pdf", "mime": "application/pdf" }]
+      },
+      "spec": {
+        "title": null,
+        "body": "Trip\n\n- [ ] passport",
+        "pinned": true,
+        "archived": false,
+        "trashed": false,
+        "remind_at": "2026-11-01T09:00:00Z",
+        "recurrence": "weekly",
+        "labels": ["travel"],
+        "items": [],
+        "attachments": [{ "file": "attachments/6ba7b810/0/ticket.pdf", "mime": "application/pdf" }],
+        "created_at": "2026-07-19T00:00:00Z",
+        "updated_at": "2026-07-20T08:30:00.250Z"
+      }
+    },
+    {
+      "about": "An export from before M304 carries a title and a separate items array; an unknown recurrence is dropped.",
+      "note": {
+        "title": "shopping",
+        "body": "",
+        "recurrence": "fortnightly",
+        "items": [{ "text": "milk", "checked": true }]
+      },
+      "spec": {
+        "title": "shopping",
+        "body": "",
+        "pinned": false,
+        "archived": false,
+        "trashed": false,
+        "remind_at": null,
+        "recurrence": null,
+        "labels": [],
+        "items": [{ "text": "milk", "checked": true }],
+        "attachments": [],
+        "created_at": null,
+        "updated_at": null
+      }
+    }
+  ],
+  "export": {
+    "document": ["app", "version", "exported_at", "labels", "notes"],
+    "note": [
+      "id",
+      "display_title",
+      "body",
+      "pinned",
+      "archived",
+      "remind_at",
+      "recurrence",
+      "created_at",
+      "updated_at",
+      "labels",
+      "attachments"
+    ],
+    "label": ["name", "color"],
+    "attachment": ["file", "mime"]
+  }
+}