dashboard: guard renderers against missing-root on /flow.html (#399)

Mara hit:

  TypeError: can't access property "innerHTML", root is null
      renderContainers app.js:666
      applyContainerStateChanged app.js:484
      container_state_changed app.js:2306

…on /flow.html. The same bundled `app.js` runs on both /index.html
(dashboard, has the tab panes) and /flow.html (flow page, has only
the broker terminal). SSE events arrive on every page —
`container_state_changed` / `tombstones_changed` / `approval_*` /
`question_*` route through their corresponding renderers, which
then `root.innerHTML = ''` on `$('section-id')` and crash when the
section isn't in the DOM.

The convention is already "no-op when the target DOM doesn't
exist" — `renderMetaInputs`, `renderRebuildQueue`, `renderReminders`
all guard with `if (!root) return;` at the top. Bring the rest in
line:

- `renderContainers`
- `renderTombstones`
- `renderQuestions`
- `renderApprovals`

`renderInbox` already handles the absent case via its
`if (root && !root.hidden)` branch — no change.

No behaviour change on /index.html. On /flow.html the failing
events silently no-op as intended (terminal renderers still
re-render the broker tail normally).

frontend/packages/dashboard/src/app.js

@@ -704,6 +704,15 @@ window.marked = marked;
 
   function renderContainers(s) {
     const root = $('containers-section');
+    // #containers-section only exists on /index.html. The same
+    // bundled app.js runs on /flow.html (which has the broker
+    // terminal but no container list) and on any future pages —
+    // `container_state_changed` SSE events arrive on every page and
+    // route through `applyContainerStateChanged → renderContainersFromState`,
+    // so without this guard the renderer throws `root is null` on
+    // every event (#399). Matches the no-op-when-target-absent
+    // convention the other renderers (renderTombstones, etc.) follow.
+    if (!root) return;
     root.innerHTML = '';
 
     // Containers come from the derived map (event-driven) rather than
@@ -1042,6 +1051,10 @@ window.marked = marked;
 
   function renderTombstones(s) {
     const root = $('tombstones-section');
+    // #tombstones-section only lives on /index.html (SYST3M tab);
+    // no-op on /flow.html and any other page that loads the shared
+    // bundle without the dashboard's tab panes (#399).
+    if (!root) return;
     root.innerHTML = '';
     if (!s.tombstones || !s.tombstones.length) {
       root.append(el('p', { class: 'empty' }, 'no kept state — clean'));
@@ -1176,6 +1189,11 @@ window.marked = marked;
   }
   function renderQuestions() {
     const root = $('questions-section');
+    // #questions-section only lives on /index.html (Y3R C4LL tab);
+    // no-op on /flow.html etc. — the bundled app.js runs everywhere
+    // and `question_added` / `question_resolved` SSE events route
+    // through here (#399).
+    if (!root) return;
     root.innerHTML = '';
     const fmt = (n) => new Date(n * 1000).toISOString().replace('T', ' ').slice(0, 19);
     const allPending = questionsState.pending;
@@ -1563,6 +1581,11 @@ window.marked = marked;
 
   function renderApprovals() {
     const root = $('approvals-section');
+    // #approvals-section only lives on /index.html (Y3R C4LL tab);
+    // no-op elsewhere — `approval_added` / `approval_resolved` SSE
+    // events route through here on every page that loads the bundle
+    // (#399).
+    if (!root) return;
     root.innerHTML = '';
 
     // Spawn request form: submitting it queues a Spawn approval that