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/hosts/uptime_widget.html

@@ -15,7 +15,12 @@
   {% for host in hosts %}
   {% set ut = uptime.get(host.id, {}) %}
   <div style="display:grid;grid-template-columns:1fr 52px 52px 52px;gap:0.35rem;align-items:center;padding:0.15rem 0;font-size:0.82rem;">
-    <span style="white-space:nowrap;overflow:hidden;text-overflow:ellipsis;">{{ host.name }}</span>
+    {% if session.user_id %}
+    <a href="/hosts/{{ host.id }}" title="{{ host.name }}"
+       style="white-space:nowrap;overflow:hidden;text-overflow:ellipsis;color:inherit;text-decoration:none;">{{ host.name }}</a>
+    {% else %}
+    <span style="white-space:nowrap;overflow:hidden;text-overflow:ellipsis;" title="{{ host.name }}">{{ host.name }}</span>
+    {% endif %}
     {% for window in ["24h", "7d", "30d"] %}
     {% set pct = ut.get(window) %}
     <span style="text-align:center;font-family:ui-monospace,monospace;font-size:0.78rem;">