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.

2 files changed, 27 insertions, 1 deletions

diff --git a/src/app/App.svelte b/src/app/App.svelte
index 7e5ac7d..e4e5bbc 100644
--- a/src/app/App.svelte
+++ b/src/app/App.svelte
@@ -296,6 +296,10 @@
 		store.queueMessage(text);
 	}
 
+	function handleCancelQueuedMessage(messageId: string) {
+		store.cancelQueuedMessage(messageId);
+	}
+
 	function handleStop() {
 		store.stopGeneration();
 	}
@@ -625,7 +629,11 @@
 			     the generic SurfaceView (dispatches on rendererId, never surface id);
 			     only shown when the queue is non-empty — an idle queue is hidden. -->
 			<div class="px-4 pt-2">
-				<SurfaceView spec={messageQueueSpec} onInvoke={handleInvoke} />
+				<SurfaceView
+					spec={messageQueueSpec}
+					onInvoke={handleInvoke}
+					onCancelQueuedMessage={handleCancelQueuedMessage}
+				/>
 			</div>
 		{/if}
 
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 22b0a25..87d2bfb 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -206,6 +206,15 @@ export interface AppStore {
    * wants to add input — the server owns the idle-vs-generating decision.
    */
   queueMessage(text: string): void;
+  /**
+   * Cancel (remove) a single queued steering message by id so it never runs
+   * (`chat.queue.cancel` WS op). Fire-and-forget + idempotent: the
+   * message-queue surface update reconciles the queue UI (the cancelled message
+   * leaves the snapshot). Targets the focused conversation's queue. The caller
+   * optimistically hides the row; a cancel of an already-drained / unknown
+   * message is a silent server no-op (nothing to roll back).
+   */
+  cancelQueuedMessage(messageId: string): void;
   selectModel(model: string): void;
   newDraft(): void;
   /** Switch the active workspace (on route change) + reset to a fresh draft in it. */
@@ -1357,6 +1366,15 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
       activeChat.queueMessage(text);
     },
 
+    cancelQueuedMessage(messageId: string): void {
+      // Fire-and-forget + idempotent. The message-queue surface (conversation-
+      // scoped) reconciles the queue UI — the cancelled row leaves the snapshot.
+      // A cancel of an already-drained / unknown message is a silent no-op, so
+      // there is no local state to roll back. Delegates to the focused
+      // conversation's chat store, which owns the conversationId + transport.
+      activeChat.cancelQueuedMessage(messageId);
+    },
+
     selectModel(model: string): void {
       activeModel = model;
       const activeId = tabsStore.activeConversationId;