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/base.html

@@ -164,6 +164,7 @@ textarea { resize: vertical; }
 .ping-row:last-child { border-bottom:none; }
 .ping-meta { min-width:120px; max-width:180px; flex-shrink:1; overflow:hidden; }
 .ping-name { font-size:0.875rem; font-weight:500; white-space:nowrap; overflow:hidden; text-overflow:ellipsis; display:block; }
+a.ping-name:hover { text-decoration:underline; }
 .ping-addr { display:block; color:var(--text-muted); font-size:0.75rem; white-space:nowrap; overflow:hidden; text-overflow:ellipsis; }
 .ping-pills { display:flex; gap:2px; flex:1; min-width:0; overflow:hidden; align-items:center; }
 .pill { display:inline-block; width:7px; height:22px; border-radius:2px; cursor:default; transition:opacity .1s; }