feat: OpenClaw session import with sorted history

Backend:
- New OpenClawEndpoint with GET /api/openclaw/sessions and POST /api/openclaw/import
- Lists all OpenClaw sessions with preview messages, model, summary
- Imports user/assistant text messages (skips tool-use/tool-result)
- Cleans user messages (removes webchat prefixes, cron tags, conversation info blocks)
- Re-import support: deletes previous openclaw-import chains before importing
- New DatabaseService methods: CreateChainForImport, CreateMessageForImport, DeleteOpenClawImport
- Added SortKey to MessageRecord (computed: openclaw-import gets id-10B, else id)

History sorting:
- OpenClaw-imported messages always appear before native messages
- Sort by CASE WHEN tag='openclaw-import' THEN id-10000000000 ELSE id END
- Cursor-based paging updated from beforeId/afterId to beforeSortKey/afterSortKey
- All history queries (GetLatestActive, Before, After, CountBefore) updated

Frontend:
- Dashboard: new 'OpenClaw Import' menu item in session card dropdown
- Modal: shows OpenClaw sessions with metadata, summary, preview messages
- Imported messages display blue 'OC' badge in chat
- API functions: apiGetOpenclawSessions, apiImportOpenclawSession
- History paging updated to use sortKey cursors

Models/MessageRecord.cs

@@ -17,6 +17,7 @@ public class MessageRecord
     public bool IsMarked { get; set; }       // als wichtig markiert
     public string DeletedOn { get; set; }    // null = aktiv, sonst Zeitstempel der Löschung
     public string CreatedAt { get; set; }
+    public long SortKey { get; set; }        // Computed: openclaw-import messages get id - 10B, else id
 }
 
 public class AttachmentRecord