feat(learn): exporting shows its steps, counts and percent, with Abort

Exporting a studyset said "Exporting…" in a snack bar until the place to
keep it was asked. It now shows a sheet with its steps (reading the
content, gathering files for an archive, packing), how many of the step's
items are done and the percent of all of it, and Abort, which keeps
nothing (decided 9 Nov 2026); then asks where to keep the file as before.

Both exporters check between studysets, and the archive's between files;
packing runs on its isolate to its end, its result dropped once aborted.
ExportCubit runs an export's work behind the sheet (runExport), which
says how it ended: made, failed (a file missing named as before) or
aborted.

Whats-New: Exporting shows its steps and how far it has come, and Abort stops it.
Whats-New-De: Der Export zeigt seine Schritte und wie weit er ist, und „Abbrechen“ beendet ihn.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/test/support/test_dependencies.dart

@@ -143,6 +143,7 @@ Dependencies testDependencies({
   SoundPlayer? sounds,
   FilePicker? files,
   FileSaver? fileSaver,
+  ExportArchiver? exportArchiver,
   YoutubeVideos? videos,
   PdfEngine? pdfEngine,
   SettingsGateway? settingsGateway,
@@ -255,7 +256,7 @@ Dependencies testDependencies({
     pdfEngine: engine,
     ankiPackages: InlineAnkiPackageReader(),
     archives: InlineArchiveReader(),
-    exportArchiver: InlineExportArchiver(),
+    exportArchiver: exportArchiver ?? InlineExportArchiver(),
     deviceSettingsStore:
         deviceSettingsStore ?? MemoryDeviceSettingsStore(deviceSettings),
     deviceSettings: deviceSettings,