Add smart combined album asset redistribution + fix locale string

Core library:
- New combine_album_assets() in asset_utils.py: smart redistribution
  of unused quota when albums return fewer assets than their share.
  Two-pass algorithm: even split then redistribute remainder.
- 6 new tests (56 total passing).

Frontend:
- Fix "leave empty to keep current" not localized in server edit.

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

frontend/src/lib/i18n/en.json

@@ -48,6 +48,7 @@
 		"url": "Immich URL",
 		"urlPlaceholder": "http://immich:2283",
 		"apiKey": "API Key",
+		"apiKeyKeep": "API Key (leave empty to keep current)",
 		"connecting": "Connecting...",
 		"noServers": "No servers configured yet.",
 		"delete": "Delete",