Fix misleading memory usage for VMs without guest-side stats

Windows VMs only report balloon current/max/rss, not available/unused.
Previously this fell through to RSS which showed ~100% usage.
Now VMs without balloon.available show allocated RAM without a usage bar.

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

app.py

@@ -216,13 +216,12 @@ def get_vm_live_stats():
             balloon_unused = s.get("balloon_unused", 0)
             balloon_rss = s.get("balloon_rss", 0)
 
-            if balloon_avail > 0:
+            if balloon_avail > 0 and balloon_unused >= 0:
                 mem_total = balloon_avail // 1024  # KiB to MB
                 mem_used = (balloon_avail - balloon_unused) // 1024
-            elif balloon_rss > 0:
-                mem_total = 0
-                mem_used = balloon_rss // 1024
             else:
+                # No guest-side stats (e.g. Windows without full guest agent).
+                # Mark as unavailable — frontend will show allocated RAM only.
                 mem_total = 0
                 mem_used = 0