fix: UX audit improvements - dark theme, structured errors, heartbeat, health check

Phase 1 - UI Theme Consistency:
- Chat messages now use consistent dark theme (removed jarring white bg)
- Plan cards restyled with rounded borders, fills, colored status badges
- Timeline entries use humanist sans-serif instead of monospace
- Error messages now structured (icon + title + detail + action link)
- Input area unified with dark theme cohesion

Phase 2 - UX Flow:
- Added contextual placeholder text per agent mode in textarea
- Added visual mode indicator badge (Chat/Planning/Writing)
- Simplified welcome screen (single 'Continue' + collapsible history)
- Added slash command/mention discovery hint in empty input
- Added write confirmation when editor has existing content
- Added 30s streaming heartbeat (reassurance when model is slow)

Phase 3 - Error Handling:
- Added DB table health check on sidebar init
- Improved 'no API key' error with settings link
- Shows in-chat warning when provider fallback triggers
- Auto-fallback to registry fallback model on unavailability
- isLoading always resets via try/finally pattern

includes/class-gutenberg-sidebar.php

@@ -243,6 +243,9 @@ class WP_Agentic_Writer_Gutenberg_Sidebar {
 		// Get settings for JS.
 		$settings = $this->get_settings_for_js();
 
+		// Health check: verify DB table and API key exist
+		$health = $this->run_health_check();
+
 		// Localize script with data.
 		$data = array(
 			'apiUrl' => rest_url( 'wp-agentic-writer/v1' ),
@@ -252,11 +255,48 @@ class WP_Agentic_Writer_Gutenberg_Sidebar {
 			'version' => WP_AGENTIC_WRITER_VERSION,
 			'debug' => defined( 'SCRIPT_DEBUG' ) && SCRIPT_DEBUG,
 			'pluginUrl' => plugin_dir_url( dirname( __FILE__ ) ),
+			'health' => $health,
 		);
 
 		wp_localize_script( 'wp-agentic-writer-sidebar', 'wpAgenticWriter', $data );
 	}
 
+	/**
+	 * Run health check for sidebar initialization.
+	 *
+	 * @since 0.2.4
+	 * @return array Health status.
+	 */
+	private function run_health_check() {
+		global $wpdb;
+		$table_name = $wpdb->prefix . 'wpaw_conversations';
+		$table_exists = $wpdb->get_var( $wpdb->prepare( 'SHOW TABLES LIKE %s', $table_name ) ) === $table_name;
+
+		$settings = get_option( 'wp_agentic_writer_settings', array() );
+		$has_api_key = ! empty( $settings['openrouter_api_key'] );
+
+		$issues = array();
+		if ( ! $table_exists ) {
+			$issues[] = array(
+				'type' => 'db_table_missing',
+				'message' => 'Conversation table not found. Please deactivate and reactivate the plugin.',
+			);
+		}
+		if ( ! $has_api_key ) {
+			$issues[] = array(
+				'type' => 'no_api_key',
+				'message' => 'API key not configured. Add your OpenRouter key in settings.',
+				'actionUrl' => admin_url( 'options-general.php?page=wp-agentic-writer-settings' ),
+				'actionLabel' => 'Open Settings',
+			);
+		}
+
+		return array(
+			'ok' => empty( $issues ),
+			'issues' => $issues,
+		);
+	}
+
 	/**
 	 * Get settings for JavaScript.
 	 *