fix(backend): align DeriveAgentStatus tail buffer with scanner max line size

CodeRabbit correctly identified that DeriveAgentStatus was reading only 64KB
from the file tail, which could truncate large events now that scannerMaxLineSize
is 10MB. This would cause JSON parsing failures and return empty status.

Changes:
- Increase maxTailBytes from 64KB to 2x scannerMaxLineSize (20MB)
- This ensures we can read at least one complete max-sized event plus additional
  events for proper status derivation
- Keeps the value in sync with scannerMaxLineSize for future changes

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Gkrumbach07

components/backend/websocket/agui_store.go

@@ -219,8 +219,9 @@ func DeriveAgentStatus(sessionID string) string {
 	path := fmt.Sprintf("%s/sessions/%s/agui-events.jsonl", StateBaseDir, sessionID)
 
 	// Read only the tail of the file to avoid loading entire event log into memory.
-	// 64KB is sufficient for recent lifecycle events (scanning backwards).
-	const maxTailBytes = 64 * 1024
+	// Use 2x scannerMaxLineSize to ensure we can read at least one complete max-sized
+	// event line plus additional events for proper status derivation.
+	maxTailBytes := int64(scannerMaxLineSize * 2)
 
 	file, err := os.Open(path)
 	if err != nil {