docs: migrate tabs.js topology-tree prose to docs/web-ui.md (#712 pattern check)

First slice of #712 (dashboard SPA frontend → docs/). Shrinks the
buildAgentTree + treePrefixDom comment blocks in tabs.js to one-line
docs/ references; adds a new 'Topology tree' subsection to
docs/web-ui.md under '### Container row' carrying the substantive
prose.

Net effect on this region:
- 6 #NNN tracking cookies dropped (#363, #361, #388)
- 30 lines of substantive design comments collapsed to 6 lines of
  references in tabs.js
- 35 new lines in docs/web-ui.md (the migrated prose, restructured
  into a single coherent subsection rather than two adjacent comment
  blocks)
- functional code unchanged; build clean

Deliberately small pattern-check (one cohesive subsystem) before
expanding to the rest of #712 (~140 more refs across tabs.js +
dashboard.css + index.html + common.js + flow.js). Reviewers can
sanity-check the shape (where prose lands, how short the in-code
ref becomes, whether section nesting reads well) before I scale.

refs #712

docs/web-ui.md

@@ -425,6 +425,41 @@ body sits to the right with three stacked lines
 agent is stale. Banner pulses on each broker SSE event
 (`pulseBanner` with a 4s grace timer).
 
+#### Topology tree
+
+Container rows render as a forest, not a flat list — each agent
+sits indented under its declared parent. `tabs.js::buildAgentTree`
+walks `ContainerView.parent` for every container in the snapshot
+and produces a render order with per-row depth + sibling-position
+info:
+
+- Top-level rows are agents with `parent = null` OR a parent that
+  doesn't appear in the container map (orphans get hoisted to root
+  so they're still visible).
+- Within each level children sort alphabetically by name; roots
+  likewise.
+- Cycle safety: any container not reached during the root-walk is
+  appended at the end as a root, so no agent ever silently
+  disappears from the list when the topology JSON is malformed.
+- The pre-topology rendering shape (every container at depth 0,
+  flat list) collapses to the same visual today when no parent
+  field is set — bit-identical fallback path.
+
+The per-row prefix column (`.tree-prefix`) is **DOM-painted, not
+text-glyph-painted**. Each indent lane is its own positioned
+`<span>` so CSS can draw full-height vertical bars that bridge the
+gap between sibling rows; using text box-drawing characters
+(`├─`, `└─`, `│  `) only paints one text-line tall and leaves
+visible breaks between the taller-than-one-line container cards.
+The bars come in two flavours: continuation (the ancestor's
+subtree extends below this row → vertical line top→bottom) or
+blank (ancestor was the last sibling at its level → no line
+needed). The joint at the row's own depth column is `├` (more
+siblings below) or `└` (last sibling at this depth — vertical
+stops at the row's icon midline).
+
+
+
 ### Selection bar
 
 When one or more agents are selected (via icon click), a sticky