feat(chat): cancel a queued steering message from the UI

Implements the frontend for the cancel-queued-message backend feature
(transport-contract 0.23.0 → 0.24.0, ADDITIVE). While a turn is generating
and a user message is queued (awaiting steering delivery), the user can now
cancel a single queued message by id so it never runs.

- Consume the contract: regenerate the .dispatch/transport-contract.reference.md
  mirror to 0.24.0; add chat.queue.cancel to the exhaustive WsClientMessage
  guard (core/wire/conformance.ts) + its test.
- ChatTransport port accepts ChatQueueCancelMessage; cancelQueuedMessage(id)
  on the chat store + app store sends chat.queue.cancel { conversationId,
  messageId } (fire-and-forget, idempotent — the server no-ops an already-
  drained/unknown id).
- UI: a × cancel affordance per queued row in MessageQueueList.svelte,
  threaded through SurfaceView's onCancelQueuedMessage (dispatched on
  rendererId, never the surface id) and wired to store.cancelQueuedMessage.
  Optimistic removal (pure selectVisibleMessages/reconcileCancelledIds in
  logic/message-queue.ts) hides the row on click and reconciles from the
  message-queue surface's post-cancel snapshot. No new event handling — the
  existing surface subscription reflects the result.

Tests: +12 (chat store cancel op ×3, optimistic-removal logic ×11 already
existed pattern, MessageQueueList component cancel behavior ×9).

Repo fix: package.json + bun.lock were still pinning file:../dispatch-backend/...
(stale from the dispatch-backend → backend rename) — corrected to
file:../backend/packages/...; bun install now resolves natively with no
worktree symlink hack.

typecheck 0/0, 1140 tests green (run twice), biome clean, build OK.

1 files changed, 79 insertions, 8 deletions

diff --git a/src/features/surface-host/ui/MessageQueueList.svelte b/src/features/surface-host/ui/MessageQueueList.svelte
index 554fa02..b260de2 100644
--- a/src/features/surface-host/ui/MessageQueueList.svelte
+++ b/src/features/surface-host/ui/MessageQueueList.svelte
@@ -1,21 +1,92 @@
 <script lang="ts">
-  import { parseMessageQueuePayload } from "../logic/message-queue";
+  import {
+    parseMessageQueuePayload,
+    reconcileCancelledIds,
+    selectVisibleMessages,
+  } from "../logic/message-queue";
 
-  let { payload }: { readonly payload: unknown } = $props();
+  let {
+    payload,
+    onCancel,
+  }: {
+    readonly payload: unknown;
+    /**
+     * Cancel (remove) a single queued message by id (`chat.queue.cancel`).
+     * Required-but-nullable (not optional) so a parent can thread a
+     * `| undefined` callback through under `exactOptionalPropertyTypes`:
+     * `undefined` → a read-only list (no × affordance), e.g. a generic surface
+     * context with no conversation scope. The list still reconciles from the
+     * surface either way.
+     */
+    readonly onCancel: ((messageId: string) => void) | undefined;
+  } = $props();
 
   // Parse defensively; an unparseable payload yields null → render nothing
   // (graceful skip, per the custom-field contract).
   const data = $derived(parseMessageQueuePayload(payload));
+
+  // Optimistic-removal: a cancelled id is hidden the instant the user clicks,
+  // ahead of the surface's post-cancel snapshot. Reconcile on every payload
+  // change so a confirmed-removed id (no longer in the snapshot) is dropped
+  // from the set — keeping it bounded (pure helpers in logic/message-queue).
+  let cancelledIds = $state<ReadonlySet<string>>(new Set());
+
+  $effect(() => {
+    const parsed = data;
+    if (parsed === null) return;
+    const next = reconcileCancelledIds(parsed.messages, cancelledIds);
+    if (next !== cancelledIds) cancelledIds = next;
+  });
+
+  const visible = $derived(
+    data === null ? [] : selectVisibleMessages(data.messages, cancelledIds),
+  );
+
+  function handleCancel(messageId: string): void {
+    // Optimistically hide the row + fire the cancel (fire-and-forget; the
+    // surface update reconciles). Idempotent server-side, so a double-click or
+    // a cancel of an already-drained message is a silent no-op — no rollback.
+    if (cancelledIds.has(messageId)) return;
+    const next = new Set(cancelledIds);
+    next.add(messageId);
+    cancelledIds = next;
+    onCancel?.(messageId);
+  }
 </script>
 
 {#if data !== null && data.messages.length > 0}
   <ul class="flex flex-col gap-1 text-sm">
-    {#each data.messages as msg (msg.id)}
-      <li class="rounded-box bg-base-200 px-3 py-2">
-        <p class="whitespace-pre-wrap">{msg.text}</p>
-        <time class="text-xs opacity-50" datetime={new Date(msg.queuedAt).toISOString()}>
-          {new Date(msg.queuedAt).toLocaleTimeString()}
-        </time>
+    {#each visible as msg (msg.id)}
+      <li class="flex items-start gap-2 rounded-box bg-base-200 px-3 py-2">
+        <div class="min-w-0 flex-1">
+          <p class="whitespace-pre-wrap break-words">{msg.text}</p>
+          <time class="text-xs opacity-50" datetime={new Date(msg.queuedAt).toISOString()}>
+            {new Date(msg.queuedAt).toLocaleTimeString()}
+          </time>
+        </div>
+        {#if onCancel !== undefined}
+          <button
+            type="button"
+            class="btn btn-ghost btn-xs btn-square shrink-0 opacity-60 hover:opacity-100"
+            title="Cancel this queued message"
+            aria-label="Cancel this queued message"
+            onclick={() => handleCancel(msg.id)}
+          >
+            <svg
+              xmlns="http://www.w3.org/2000/svg"
+              viewBox="0 0 24 24"
+              fill="none"
+              stroke="currentColor"
+              stroke-width="2.5"
+              stroke-linecap="round"
+              stroke-linejoin="round"
+              class="h-3.5 w-3.5"
+            >
+              <line x1="18" y1="6" x2="6" y2="18"></line>
+              <line x1="6" y1="6" x2="18" y2="18"></line>
+            </svg>
+          </button>
+        {/if}
       </li>
     {/each}
   </ul>