dashboard: render agent topology as a tree in the container list

Closes #363 (frontend half of milestone #361). Consumes the
`ContainerView.parent: Option<String>` field landing in damocles'
backend slice — when present, the container list renders depth-first
with sibling-position tree glyphs (├─ / └─ joints + │ continuation
columns). When absent (pre-#361 state — every container's `parent` is
None) the tree collapses to a flat list with no glyphs and no indent
— bit-identical to the legacy render.

## Tree shape

- Roots: containers whose `parent` is None OR whose parent name isn't
  in the current container set (orphan tolerance).
- Sibling ordering: alphabetical by name within each level (matches
  damocles' wire spec at #363#issuecomment-3356).
- Cycle safety: any container not reached via the root-walk gets
  emitted as a root at the end — no agent ever silently disappears
  from the list when the topology is malformed.
- Tree glyphs: ancestor at depth d contributes a │ continuation column
  when that ancestor has more siblings below; otherwise a 3-space gap.
  The joint is ├─ for non-last siblings, └─ for the last child.
- Depth-0 ancestor column is suppressed: roots already separate
  visually as top-level rows, no need for a column 0 vertical line.

## DOM / CSS

- New `buildAgentTree(containers)` + `treePrefix(node)` helpers in
  app.js. The render loop walks the tree-ordered list instead of the
  legacy alphabetical containers array.
- Each container row gets `data-depth=N` (only when N > 0) and a
  `<span class="tree-prefix">…</span>` prepended (absolute-positioned
  into the row's left margin so the existing flex icon/body layout
  isn't disrupted).
- CSS: per-depth `margin-left` step rules for depths 1-6 (hardcoded
  rather than typed-attr() because CSS Values 5 is Chromium-only as
  of 2026). 6 depths cover any reasonable hive topology with
  headroom; deeper agents render at depth 6 indent without further
  step — visually clamps gracefully.
- `.tree-prefix` rendered with `var(--purple-dim)` so the structural
  lines read as supporting chrome, not as content.

## Validation

- `npm run build` clean. Bundle deltas: dashboard app.js
  +1.8kb (tree builder + treePrefix + render-loop tweak),
  dashboard.css +0.4kb (tree-prefix + per-depth indent rules).
- The render is a no-op until `ContainerView.parent` is populated —
  validation in production deferred to once damocles' meta-topology
  field lands. The pre-#361 path (every parent=None) is exercised
  by every existing dashboard load.
- Forward-compatible with damocles' design pivot at #364 (topology
  source moved from agent.nix to meta/topology.json). The wire shape
  on ContainerView is unchanged from the frontend's perspective — the
  field is just sourced from a different backend store.

frontend/packages/dashboard/src/dashboard.css

@@ -82,6 +82,36 @@ a:hover {
   background: rgba(24, 24, 37, 0.55);
   transition: opacity 200ms ease, border-color 200ms ease;
 }
+/* Topology indent (#363). Each depth level shifts the row right by one
+   step; the .tree-prefix span (drawn by app.js::treePrefix) carries
+   the ├─ / └─ glyph and any continuation lines that thread through
+   ancestor columns. When every container has parent=null (pre-#361
+   state) `[data-depth]` is absent on every row and these rules are
+   no-ops — the layout reads exactly like the legacy flat list.
+   Per-depth indent: hardcoded steps for 6 levels (sufficient for any
+   plausible hive topology) — the typed `attr()` function from CSS
+   Values 5 would collapse this to one rule, but browser support is
+   still partial (Chromium-only as of 2026). */
+.container-row[data-depth] { position: relative; }
+.container-row[data-depth="1"] { margin-left: 1.8em; }
+.container-row[data-depth="2"] { margin-left: 3.6em; }
+.container-row[data-depth="3"] { margin-left: 5.4em; }
+.container-row[data-depth="4"] { margin-left: 7.2em; }
+.container-row[data-depth="5"] { margin-left: 9em; }
+.container-row[data-depth="6"] { margin-left: 10.8em; }
+.container-row .tree-prefix {
+  /* Tree-line glyphs sit in the left margin so the rest of the
+     container card layout (icon + body flex split) stays unchanged.
+     Mono font for column alignment with the ├─ / └─ / │  bricks. */
+  position: absolute;
+  left: -1.6em;
+  top: 0.6em;
+  color: var(--purple-dim);
+  font-family: inherit;
+  white-space: pre;
+  pointer-events: none;
+  user-select: none;
+}
 /* Live cards get the icon-left / body-right split; tombstone rows keep
    the plain stacked block layout. The icon is a background-image div
    with no intrinsic size, so its load state can never reflow the row