feat(dashboard): phase A widget clarity — threshold colours, host links, tooltips

Milestone 72 phase A (clarity wins, no schema change):
- Add shared metric_style() macro in _macros.html: colours a numeric metric
  amber (>=warn) / red (>=crit) on the value ITSELF, not just the status dot.
  Defaults 80/90 for percentage gauges; load /core uses warn 80 / crit 100.
  Applied to CPU/mem/disk across host_agent panel + fleet widget + the unified
  hosts-overview widget.
- Link every host reference to the host hub (/hosts/<id>) in ping, dns,
  hosts-overview, host_agent fleet, and uptime widgets; status widget entries
  link to their own detail_url. All guarded on session.user_id so the public
  share view degrades to plain text (the bare href carries no share token).
- Fix truncation: title= tooltips on all names that ellipsis, plus a hover
  underline affordance on the now-clickable ping-name links.

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

steward/templates/_macros.html

@@ -16,3 +16,16 @@
   {%- endfor -%}
 </nav>
 {%- endmacro %}
+
+{# Threshold emphasis for a numeric metric (CPU/mem/disk %, etc).
+   Returns an inline-style fragment to drop into a span's style="": amber +
+   semibold at >= warn, red + bold at >= crit, empty (inherit) when the value
+   is normal or None. This makes the FAILING METRIC ITSELF stand out — not just
+   the status dot. Defaults suit percentage gauges (warn 80 / crit 90); pass
+   explicit warn/crit for other scales. Keep thresholds here so they live in one
+   place rather than hardcoded per template. #}
+{% macro metric_style(value, warn=80, crit=90) -%}
+{%- 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 %}