Phase 5: Frontend - Use key as primary series identifier

- Updated app.js to use 'key' as primary series identifier
  - selectedSeries Set now uses key instead of folder
  - createSerieCard() uses data-key attribute for identification
  - toggleSerieSelection() uses key for lookups
  - downloadSelected() iterates with key values
  - updateSelectionUI() and toggleSelectAll() use key

- Updated WebSocket service tests
  - Tests now include key and folder in broadcast data
  - Verified both fields are included in messages

- No changes needed for queue.js and other JS files
  - They use download item IDs correctly, not series identifiers

- No template changes needed
  - Series cards rendered dynamically in app.js

All 996 tests passing

data/config_backups/config_backup_20251128_161248.json

@@ -0,0 +1,24 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$VCqllLL2vldKyTmHkJIyZg$jNllpzlpENdgCslmS.tG.PGxRZ9pUnrqFEQFveDEcYk",
+    "anime_directory": "/mnt/server/serien/Serien/"
+  },
+  "version": "1.0.0"
+}

data/config_backups/config_backup_20251128_161448.json

@@ -0,0 +1,24 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$3/t/7733PkdoTckZQyildA$Nz9SdX2ZgqBwyzhQ9FGNcnzG1X.TW9oce3sDxJbVSdY",
+    "anime_directory": "/mnt/server/serien/Serien/"
+  },
+  "version": "1.0.0"
+}