feat: mark cell as failed when decrementing if value is already zero

src/routes/(app)/habits/CounterHabitButton.svelte

@@ -114,6 +114,17 @@
 		} else {
 			const oldCount = count;
 			if (e.shiftKey) {
+				// If already at zero, mark as failed                                                                                   
+    	        if (count === 0) {                                                                                                      
+    	          const dateObj = new Date(date + 'T00:00:00.000Z');                                                                    
+    	          try {                                                                                                                 
+    	            await habitService.upsertCompletion(habitId, dateObj, 0, true);                                                     
+    	            return;                                                                                                             
+    	          } catch (error) {                                                                                                     
+    	            console.error('Failed to mark habit as failed:', error);                                                            
+    	            return;                                                                                                             
+    	          }                                                                                                                     
+    	        } 
 				// Decrement but don't go below 0 - animate to 0 if needed
 				const newCount = count - increment;
 				count = newCount < 0 ? 0 : newCount;