mem.used on Linux includes reclaimable buff/cache, which caused the
System Information page to show 97% memory usage on a 64GB machine
that actually had 53GB available. The warning banner fired at >90%
creating a false alarm.
Now uses (total - available) for the gauge, percentage, and displayed
values. Also renames "Free RAM" to "Available RAM" using mem.available
instead of mem.free, since free is misleadingly small on Linux (it
excludes reclaimable cache).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>