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>

desktop/src-tauri/src/commands/local.rs

@@ -7,6 +7,7 @@ use serde_json::Value;
 use tauri::State;
 
 use inkwell_core::local::models::*;
+use inkwell_core::local::portable::{self, ImportSummary};
 use inkwell_core::local::retention;
 use inkwell_core::local::store;
 use inkwell_core::local::Db;
@@ -150,6 +151,60 @@ pub async fn notes_add_attachment(
     store::add_attachment(&conn, &blobs, note_id, &filename, mime, bytes)
 }
 
+/// Write every note to a zip in the Downloads folder and return where it went.
+///
+/// The same archive the server's Export makes, built here so it works with no server.
+/// Downloads rather than a Save As dialog, which would need another plugin; the path
+/// comes back so the window can say exactly where the file is. Never overwrites: a
+/// second export the same day gets `-2`, `-3`, and so on.
+#[tauri::command]
+pub async fn notes_export(
+    app: tauri::AppHandle,
+    db: State<'_, Db>,
+    blobs: State<'_, BlobStore>,
+) -> Result<String, String> {
+    use tauri::Manager;
+    let bytes = {
+        let conn = db.0.lock().map_err(|e| e.to_string())?;
+        portable::export_zip(&conn, &blobs)?
+    };
+    let dir = app
+        .path()
+        .download_dir()
+        .or_else(|_| app.path().home_dir())
+        .map_err(|e| format!("Couldn't find a folder to save to: {e}"))?;
+    let path = unused_path(&dir, &portable::export_stem());
+    std::fs::write(&path, bytes).map_err(|e| format!("Couldn't save {}: {e}", path.display()))?;
+    Ok(path.display().to_string())
+}
+
+/// `<stem>.zip`, or the first `<stem>-N.zip` that doesn't exist yet.
+fn unused_path(dir: &std::path::Path, stem: &str) -> std::path::PathBuf {
+    let first = dir.join(format!("{stem}.zip"));
+    if !first.exists() {
+        return first;
+    }
+    (2..)
+        .map(|n| dir.join(format!("{stem}-{n}.zip")))
+        .find(|p| !p.exists())
+        .unwrap_or(first)
+}
+
+/// Import an Inkwell export or a Google Keep Takeout zip. The archive is the raw IPC
+/// body, as an attachment's bytes are. Additive: nothing already here is changed.
+#[tauri::command]
+pub async fn notes_import(
+    request: tauri::ipc::Request<'_>,
+    db: State<'_, Db>,
+    blobs: State<'_, BlobStore>,
+) -> Result<ImportSummary, String> {
+    let tauri::ipc::InvokeBody::Raw(bytes) = request.body() else {
+        return Err("The file's contents didn't arrive.".to_string());
+    };
+    let conn = db.0.lock().map_err(|e| e.to_string())?;
+    portable::import_zip(&conn, &blobs, bytes)
+}
+
 #[tauri::command]
 pub fn notes_delete_attachment(
     id: String,

desktop/src-tauri/src/lib.rs

@@ -178,6 +178,8 @@ pub fn run() {
             commands::local::notes_update_item,
             commands::local::notes_delete_item,
             commands::local::notes_add_attachment,
+            commands::local::notes_export,
+            commands::local::notes_import,
             commands::local::notes_delete_attachment,
             commands::local::notes_delete_preview,
             commands::local::notes_reorder,