R-889: every disk percent is df's (used / (used + available)); the tile says Rendszer, not Rendszer (/) (09 decision 138)

The box divided used by the whole filesystem, so the 5 % reserved for root made a full disk read ~5 points low
(61.8 of 68.7 GB = 90 % where df said 95 %) and the fill alarms fire late. One function, DFUsedPercent, now serves
GetDiskUsage, readDiskUsage, the recovery-unit headroom projection and the deploy page's free percent. Tests use
numbers measured on demo-hp 9201 (/mnt/sys_drive: old 21.5 %, df 23 %); a source scan refuses a percent divided by
the whole filesystem (red-proved by putting the old line back). The label measured the docker data volume, not /.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/web/testdata/i18n_parity/dashboard_diskcrit.html

@@ -224,7 +224,7 @@
         
         <div class="system-info-item meter crit">
             <div class="system-info-header">
-                <span class="system-info-label">Rendszer (/)</span>
+                <span class="system-info-label">Rendszer</span>
                 <span class="system-info-value">97.0 GB / 100 GB (97%)</span>
             </div>
             <div class="meter-track">