fix: writing stuck - handle empty model response + no-divider fallback + timeline cleanup

Root causes of writing getting stuck:
1. Model returns empty response for a section → now detected early with
   actionable error message including model name
2. Model responds but without ~~~ARTICLE~~~ divider (happens with fallback
   models like Gemini) → now treats entire response as markdown content
3. Stream ends without 'complete' event (error/exit in PHP) → JS timeline
   entries lingered as 'active' forever. Now deactivated on stream close.
4. Error messages in execution flow now use structured formatAiErrorMessage
   with retry button instead of raw text

Also: deactivateActiveTimelineEntries called in catch block so errors
properly clear the 'Writing section X' status indicator.

includes/class-gutenberg-sidebar.php

@@ -3343,6 +3343,42 @@ Remember: You MUST include the ~~~ARTICLE~~~ divider to separate your conversati
 						exit;
 					}
 
+					// Handle empty response from model
+					if ( empty( trim( (string) $accumulated_content ) ) ) {
+						$model_used = $response['model'] ?? 'unknown';
+						wpaw_debug_log( "Section writing got empty response from model: {$model_used}" );
+						echo "data: " . wp_json_encode(
+							array(
+								'type'    => 'error',
+								'message' => sprintf( 'Section "%s" got an empty response from the AI model (%s). Please retry.', $heading, $model_used ),
+							)
+						) . "\n\n";
+						flush();
+						exit;
+					}
+
+					// If divider was never found, treat the entire content as markdown
+					if ( ! $divider_found ) {
+						wpaw_debug_log( 'No ~~~ARTICLE~~~ divider found in section response. Using full content as markdown.' );
+						$markdown_content = $accumulated_content;
+						// Strip any leading conversational fluff (first line if it looks like a note)
+						$lines = explode( "\n", $markdown_content );
+						if ( ! empty( $lines[0] ) && ! preg_match( '/^#{1,3}\s/', $lines[0] ) && strlen( $lines[0] ) < 200 ) {
+							// First line might be a brief conversational note, skip it
+							$first_line = array_shift( $lines );
+							if ( ! empty( $first_line ) ) {
+								echo "data: " . wp_json_encode(
+									array(
+										'type'    => 'conversational',
+										'content' => trim( $first_line ),
+									)
+								) . "\n\n";
+								flush();
+							}
+							$markdown_content = implode( "\n", $lines );
+						}
+					}
+
 					$section_cost = $response['cost'] ?? 0;
 					$total_cost += $section_cost;