fix: session persistence + h3 readability + outline error messages

Session issues fixed:
- Removed auto-draft-only gate for showing unassigned sessions on new posts
  (now shows on any post that has no linked sessions)
- Auto-link unassigned session to current post when user opens it
- Added beforeunload/pagehide flush to persist messages before page close
  (prevents data loss from 700ms debounce not firing)
- Added warning log when session loads with 0 messages for debugging

UI fix:
- Override WP editor h3 shrinkage (11px/uppercase → 15px/normal/white)
- Fix h2/h4-h6 headings in response content for dark theme readability

Outline error messages:
- Separate empty response from parse failure with distinct actionable messages
- Show model name + token count on empty response for debugging
- Reassure user that parse failures are usually one-time issues

includes/class-gutenberg-sidebar.php

@@ -2296,13 +2296,25 @@ Keep sections focused and actionable. Include H2 headings only. For technical ar
 		// Debug: log the raw response
 		wpaw_debug_log( 'Plan generation raw response length: ' . strlen( $content ) );
 
+			if ( empty( trim( (string) $content ) ) ) {
+				$model_used = $response['model'] ?? 'unknown';
+				return new WP_Error(
+					'empty_response',
+					sprintf(
+						__( 'The AI model (%s) returned an empty response. Try a different planning model or simplify your topic.', 'wp-agentic-writer' ),
+						$model_used
+					),
+					array( 'status' => 500 )
+				);
+			}
+
 			if ( null === $plan_json ) {
 				wpaw_debug_log( 'extract_plan_from_response returned null. Content preview: ' . substr( $content, 0, 500 ) );
 				return new WP_Error(
 					'invalid_json',
 					sprintf(
 						/* translators: %s: model output preview */
-						__( 'The model responded, but the outline format could not be parsed. Preview: %s', 'wp-agentic-writer' ),
+						__( 'The AI responded but the outline couldn\'t be parsed as JSON. Try again — this is usually a one-time formatting issue. Preview: %s', 'wp-agentic-writer' ),
 						$this->build_model_output_preview( $content )
 					),
 					array( 'status' => 500 )
@@ -2904,6 +2916,25 @@ Keep sections focused and actionable. Include H2 headings only. For technical ar
 
 			$content = $response['content'];
 			wpaw_debug_log( 'stream_generatePlan content length: ' . strlen( $content ) );
+
+			// Handle empty response gracefully
+			if ( empty( trim( (string) $content ) ) ) {
+				$model_used = $response['model'] ?? 'unknown';
+				$input_tokens = $response['input_tokens'] ?? 0;
+				echo "data: " . wp_json_encode(
+					array(
+						'type'    => 'error',
+						'message' => sprintf(
+							'The AI model (%s) returned an empty response. This usually means the model couldn\'t process the request. Try: 1) Use a different planning model in Settings, 2) Simplify your topic, or 3) Try again. (Tokens sent: %d)',
+							$model_used,
+							$input_tokens
+						),
+					)
+				) . "\n\n";
+				flush();
+				exit;
+			}
+
 			$plan_json = $this->extract_plan_from_response( $content, $topic );
 
 			if ( null === $plan_json ) {
@@ -2912,7 +2943,7 @@ Keep sections focused and actionable. Include H2 headings only. For technical ar
 				echo "data: " . wp_json_encode(
 					array(
 						'type'    => 'error',
-						'message' => 'The model responded, but the outline format could not be parsed. Preview: ' . $preview,
+						'message' => 'The AI responded but the outline couldn\'t be parsed as JSON. This sometimes happens when the model adds extra text. Trying again usually fixes this. Preview: ' . $preview,
 					)
 				) . "\n\n";
 				flush();