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.

assets/js/sidebar.js

@@ -2912,12 +2912,20 @@
 					}
 				}
 				clearTimeout(timeout);
+				// If stream ended without a 'complete' event, deactivate lingering timeline entries
+				setMessages(prev => {
+					const hasActive = prev.some(m => m.type === 'timeline' && m.status && !['complete', 'inactive', 'stopped'].includes(m.status));
+					if (hasActive) {
+						return deactivateActiveTimelineEntries(prev);
+					}
+					return prev;
+				});
 			} catch (error) {
 				setAgentMode(currentPlanRef.current ? 'planning' : 'chat');
-				setMessages(prev => [...prev, {
+				setMessages(prev => [...deactivateActiveTimelineEntries(prev), {
 					role: 'system',
 					type: 'error',
-					content: 'Error: ' + (error.message || 'Failed to execute outline'),
+					content: formatAiErrorMessage(error, 'Failed to execute outline'),
 					canRetry: true,
 					retryType: 'execute',
 				}]);