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-openrouter-provider.php

@@ -784,7 +784,21 @@ class WP_Agentic_Writer_OpenRouter_Provider implements WP_Agentic_Writer_AI_Prov
 		// Validate model availability before making API call
 		$model_validation = $this->validate_model_availability( $model );
 		if ( is_wp_error( $model_validation ) ) {
-			return $model_validation;
+			// Auto-fallback: try registry fallback model instead of hard-failing
+			$fallback_model = WPAW_Model_Registry::get_fallback_model( $type );
+			if ( $fallback_model && $fallback_model !== $model ) {
+				$fallback_validation = $this->validate_model_availability( $fallback_model );
+				if ( true === $fallback_validation ) {
+					$model = $fallback_model;
+					if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
+						error_log( "WPAW: Model unavailable, auto-fallback to: {$fallback_model}" );
+					}
+				} else {
+					return $model_validation;
+				}
+			} else {
+				return $model_validation;
+			}
 		}
 
 		// Build request body.