Fix file locking race condition in SMART trend analysis

Check both file existence AND size > 0 before opening in r+ mode.
Previously, an empty file (0 bytes) would be opened in r+ mode, causing
json.load() to fail on the empty content.

Resolves #7

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hwmonDaemon.py

@@ -783,7 +783,10 @@ class SystemHealthMonitor:
 
             # Load historical data with file locking
             history = []
-            file_mode = 'r+' if os.path.exists(historical_file) else 'w+'
+            if os.path.exists(historical_file) and os.path.getsize(historical_file) > 0:
+                file_mode = 'r+'
+            else:
+                file_mode = 'w+'
 
             with open(historical_file, file_mode) as f:
                 # Acquire exclusive lock