Fix Maloja importer resilience: nil MBZ client, null album, error handling

Root cause of the panic at ~1,693 items: importers created
`&mbz.MusicBrainzClient{}` (nil requestQueue) instead of using the
engine's properly initialized client. When any code path called an MBZ
method, it panicked on the nil channel.

Changes:
- Pass engine's MBZ client to Maloja and Spotify importers
- Change MalojaTrack.Album to pointer type to handle null album JSON
- Continue on error instead of aborting the entire import
- Accept both Maloja export formats ("scrobbles" and "list" keys)
- Extract per-file import into importFile() with its own defer/recover
- Add progress logging every 500 items

Test fixtures:
- maloja_import_null_album_test.json (null album, valid album, empty artists)
- maloja_api_format_test.json (API "list" format)

New tests: TestImportMaloja_NullAlbum, TestImportMaloja_ApiFormat

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test_assets/maloja_api_format_test.json

@@ -0,0 +1,41 @@
+{
+    "status": "ok",
+    "list": [
+      {
+        "time": 1746434410,
+        "track": {
+          "artists": [
+            "API Format Artist"
+          ],
+          "title": "API Format Track",
+          "album": {
+            "artists": [
+              "API Format Artist"
+            ],
+            "albumtitle": "API Format Album"
+          },
+          "length": null
+        },
+        "duration": null,
+        "origin": "client:default"
+      },
+      {
+        "time": 1746434682,
+        "track": {
+          "artists": [
+            "API Format Artist"
+          ],
+          "title": "API Format Track 2",
+          "album": {
+            "artists": [
+              "API Format Artist"
+            ],
+            "albumtitle": "API Format Album"
+          },
+          "length": null
+        },
+        "duration": null,
+        "origin": "client:default"
+      }
+    ]
+}

test_assets/maloja_import_null_album_test.json

@@ -0,0 +1,52 @@
+{
+    "maloja": {
+      "export_time": 1748738994
+    },
+    "scrobbles": [
+      {
+        "time": 1746434410,
+        "track": {
+          "artists": [
+            "Null Album Artist"
+          ],
+          "title": "Track With Null Album",
+          "album": null,
+          "length": null
+        },
+        "duration": null,
+        "origin": "client:default"
+      },
+      {
+        "time": 1746434682,
+        "track": {
+          "artists": [
+            "Valid Album Artist"
+          ],
+          "title": "Track With Valid Album",
+          "album": {
+            "artists": [
+              "Valid Album Artist"
+            ],
+            "albumtitle": "My Great Album"
+          },
+          "length": null
+        },
+        "duration": null,
+        "origin": "client:default"
+      },
+      {
+        "time": 1746434900,
+        "track": {
+          "artists": [],
+          "title": "Track With Empty Artists",
+          "album": {
+            "artists": [],
+            "albumtitle": "Some Album"
+          },
+          "length": null
+        },
+        "duration": null,
+        "origin": "client:default"
+      }
+    ]
+}