dashboard: full-height square agent icon, icon-left card layout

The agent icon was a 26px <img> inline in the card head, hidden via
onerror when a stopped container's web server didn't answer — which
collapsed the slot and shifted the row.

Restructure the live container card as icon-left / body-right:
- the icon is a background-image div with aspect-ratio 1 and
  align-self stretch — full card height, square, and (being a
  background) it has no intrinsic size, so loading or failing it
  can never reflow the row;
- a failed load (stopped container) falls through to a placeholder
  fill instead of collapsing;
- the three content lines move into a .card-body column.

Tombstone rows keep the plain stacked layout (:not(.tombstone)).

closes #177

docs/web-ui.md

@@ -148,7 +148,10 @@ the previous process's socket release resolves itself.
 
 ### Container row
 
-Two-line layout (`assets/app.js::renderContainers`):
+A full-height **square agent icon** on the left (the agent's
+`/icon`, painted as a background-image div so its load state can
+never reflow the row), and the card body on the right with three
+stacked lines (`assets/app.js::renderContainers`):
 
 - Line 1: agent name (link → new tab), m1nd/ag3nt chip, status
   badges — `⊘ rate limited` (red, while the harness is parked