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>

Cargo.lock

@@ -1859,6 +1859,7 @@ name = "inkwell-core"
 version = "0.1.0"
 dependencies = [
  "chrono",
+ "flate2",
  "log",
  "openssl-sys",
  "reqwest 0.12.28",
@@ -1867,6 +1868,7 @@ dependencies = [
  "serde_json",
  "sha2",
  "uuid",
+ "zip",
 ]
 
 [[package]]
@@ -5765,6 +5767,7 @@ checksum = "caa8cd6af31c3b31c6631b8f483848b91589021b28fffe50adada48d4f4d2ed1"
 dependencies = [
  "arbitrary",
  "crc32fast",
+ "flate2",
  "indexmap 2.14.0",
  "memchr",
 ]