fix: support CSV/TXT upload and add validation for Plan to Eat import (#6360)

Plan to Eat exports CSV or TXT files directly, but the importer only accepted
ZIP archives. This caused a silent failure when users uploaded CSV files.

- Extend plantoeat_recipes() to detect ZIP vs CSV/TXT by magic bytes and
  process raw CSV/TXT files directly without requiring a ZIP wrapper
- Add _validate_archive() to return a clear error report entry when the
  uploaded file is neither a ZIP nor valid UTF-8 text
- Update frontend file input to accept .zip, .csv, and .txt
- Update i18n description to mention all accepted formats
- Add plantoeat.csv test fixture and integration tests for CSV import
  and invalid file type rejection

tests/integration_tests/recipe_migration_tests/test_recipe_migrations.py

@@ -94,6 +94,15 @@ test_cases = [
             "transFatContent",
         },
     ),
+    MigrationTestData(
+        typ=SupportedMigrations.plantoeat,
+        archive=test_data.migrations_plantoeat_csv,
+        search_slug="test-recipe",
+        nutrition_filter={
+            "unsaturatedFatContent",
+            "transFatContent",
+        },
+    ),
     MigrationTestData(
         typ=SupportedMigrations.myrecipebox,
         archive=test_data.migrations_myrecipebox,
@@ -124,6 +133,7 @@ test_ids = [
     "mealie_alpha_archive",
     "tandoor_archive",
     "plantoeat_archive",
+    "plantoeat_csv",
     "myrecipebox_csv",
     "recipekeeper_archive",
     "cookn_archive",
@@ -190,6 +200,30 @@ def test_recipe_migration(api_client: TestClient, unique_user_fn_scoped: TestUse
     # TODO: validate other types of content
 
 
+def test_plantoeat_rejects_invalid_file_type(api_client: TestClient, unique_user: TestUser) -> None:
+    # Simulate uploading a binary file (e.g. PDF) that is neither ZIP nor CSV/TXT
+    binary_content = bytes(range(256)) * 4  # arbitrary binary data that is not valid UTF-8
+    payload = {"migration_type": SupportedMigrations.plantoeat.value}
+    file_payload = {"archive": binary_content}
+
+    response = api_client.post(
+        api_routes.groups_migrations,
+        data=payload,
+        files=file_payload,
+        headers=unique_user.token,
+    )
+
+    assert response.status_code == 200
+    report_id = response.json()["id"]
+
+    response = api_client.get(api_routes.groups_reports_item_id(report_id), headers=unique_user.token)
+    assert response.status_code == 200
+    report = response.json()
+    assert report["entries"]
+    assert not report["entries"][0]["success"]
+    assert "ZIP" in report["entries"][0]["message"] or "CSV" in report["entries"][0]["message"]
+
+
 def test_bad_mealie_alpha_data_is_ignored(api_client: TestClient, unique_user: TestUser):
     with TemporaryDirectory() as tmpdir:
         with ZipFile(test_data.migrations_mealie) as zf: