feat(dashboard): threshold colors on Hosts-Overview ping + 24h uptime

Add an inverted uptime_style macro (low-is-bad mirror of metric_style) and
color the inline ping latency (warn 100ms / crit 250ms) and 24h uptime values
in the Hosts-Overview widget, which were the remaining uncolored metrics.
(Uptime/SLA + Ping widgets already colored degraded values.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/templates/_macros.html

@@ -29,3 +29,12 @@
 {%- elif value is not none and value >= warn -%}color:var(--yellow);font-weight:600;
 {%- endif -%}
 {%- endmacro %}
+
+{# Inverted emphasis for "higher is better" metrics like uptime %: amber below
+   `warn`, red below `crit`. The mirror of metric_style — same one-place-for-
+   thresholds discipline, opposite direction. Defaults suit SLA percentages. #}
+{% macro uptime_style(value, warn=99.0, crit=95.0) -%}
+{%- if value is not none and value < crit -%}color:var(--red);font-weight:700;
+{%- elif value is not none and value < warn -%}color:var(--yellow);font-weight:600;
+{%- endif -%}
+{%- endmacro %}