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

Endpoints/HistoryEndpoint.cs

@@ -43,16 +43,16 @@ public static class HistoryEndpoint
         }
 
         const int messageLimit = 500;
-        var beforeStr = ctx.Request.Query["beforeId"].FirstOrDefault();
-        var beforeId = long.TryParse(beforeStr, out var b) ? b : 0;
+        var beforeStr = ctx.Request.Query["beforeSortKey"].FirstOrDefault();
+        var beforeSortKey = long.TryParse(beforeStr, out var b) ? b : 0;
         var includeDeleted = ctx.Request.Query["includeDeleted"].FirstOrDefault() == "true";
 
         var totalCount = await db.GetActiveMessageCount(sessionId, includeDeleted);
 
         List<MessageRecord> messages;
-        if (beforeId > 0)
+        if (beforeSortKey != 0)
         {
-            messages = await db.GetActiveMessagesBeforeId(sessionId, beforeId, messageLimit, includeDeleted);
+            messages = await db.GetActiveMessagesBeforeSortKey(sessionId, beforeSortKey, messageLimit, includeDeleted);
         }
         else
         {
@@ -60,9 +60,9 @@ public static class HistoryEndpoint
         }
 
         // Zähle wie viele Nachrichten es VOR der ältesten geladenen noch gibt
-        var oldestLoadedId = messages.Count > 0 ? messages[0].Id : 0;
-        var remainingOlder = oldestLoadedId > 0
-            ? await db.GetActiveMessageCountBeforeId(sessionId, oldestLoadedId, includeDeleted)
+        var oldestLoadedSortKey = messages.Count > 0 ? messages[0].SortKey : 0;
+        var remainingOlder = oldestLoadedSortKey != 0
+            ? await db.GetActiveMessageCountBeforeSortKey(sessionId, oldestLoadedSortKey, includeDeleted)
             : 0;
 
         // Nach Chain gruppieren (Reihenfolge beibehalten)
@@ -91,8 +91,8 @@ public static class HistoryEndpoint
     private static async Task HandlePoll(HttpContext ctx, DatabaseService db)
     {
         var sessionId = ctx.Request.Query["sessionId"].FirstOrDefault();
-        var afterStr = ctx.Request.Query["afterId"].FirstOrDefault();
-        var afterId = long.TryParse(afterStr, out var a) ? a : 0;
+        var afterStr = ctx.Request.Query["afterSortKey"].FirstOrDefault();
+        var afterSortKey = long.TryParse(afterStr, out var a) ? a : 0;
         var includeDeleted = ctx.Request.Query["includeDeleted"].FirstOrDefault() == "true";
 
         if (string.IsNullOrEmpty(sessionId) || !await VerifySessionOwner(ctx, db, sessionId))
@@ -101,7 +101,7 @@ public static class HistoryEndpoint
             return;
         }
 
-        var messages = await db.GetActiveMessagesAfterId(sessionId, afterId, includeDeleted: includeDeleted);
+        var messages = await db.GetActiveMessagesAfterSortKey(sessionId, afterSortKey, includeDeleted: includeDeleted);
         await ctx.Response.WriteAsJsonAsync(messages);
     }