dashboard: flatten the agent list, drop dead topology-tree machinery (#4638)

The backend dropped the agent hierarchy's parent field, so every
container is a root and buildAgentTree/treePrefixDom could only ever
produce a single-level flat list — the .tree-prefix CSS lane rules
already matched nothing. Replaced with sortedContainerRows, a plain
alphabetical sort, and dropped the now-dead .tree-prefix/.tree-lane/
data-depth CSS and the depth/isLast/ancestorIsLast fields from the row
fingerprint and buildContainerLi's signature. No visible behavior
change - the rendered list was already flat, just via dead machinery.

Docs updated to describe the simpler implementation directly instead
of narrating the removal (kept the heading name since swarm.js still
points a comment at it).

docs/web-ui/dashboard.md

@@ -976,11 +976,14 @@ identically.)
 
 ### Topology tree
 
-Nothing nests. `swarm.js::buildAgentTree` returns every container at
-depth 0 in alphabetical order, and the row builder emits no prefix
-column for a depth-0 row — so a container row carries no indent, no
-joint and no continuation bar, and the SW4RM list reads as a flat
-roster. The `.tree-prefix` lane rules in `dashboard.css` match nothing.
+Nothing nests. `swarm.js::sortedContainerRows` returns every container
+in alphabetical order, full stop — the SW4RM list is a flat roster, no
+indent, no joint, no continuation bar. This used to be a real forest
+walk (`buildAgentTree`, keyed on each container's `parent` field) with
+a DOM-painted `.tree-prefix` indent ladder. Once the backend dropped
+the `parent` field entirely, every container became a root and the
+tree machinery could only ever produce this same flat order — so it
+was removed rather than kept as code that draws a tree with one level.
 
 ## Selection bar