refactor(app): extract composer-actions module with pure tests (#720)

Move composer cancel/queue/attachment/dispatch orchestration out of
composer.tsx into a React-free actions module. The module takes its
dependencies (send client, queue writer, stream writer, attachment
persister, image picker) as injected ports, so the new
composer-actions.test.ts can drive every action with inline fakes —
zero vi.mock of internal modules, zero JSDOM, zero React.

The old composer.test.tsx (heavy mocked component test) is removed
and replaced with composer-actions.test.ts (31 unit tests).

Also extract isWorkspaceAttachment / userAttachmentsOnly /
workspaceAttachmentToSubmitAttachment from composer-workspace-attachments.tsx
into a sibling .ts so the actions module (and composer-attachments.ts)
can use them without dragging React/RN/lucide into a pure test graph.

packages/app/src/attachments/workspace-attachment-utils.ts

@@ -0,0 +1,35 @@
+import type {
+  ComposerAttachment,
+  UserComposerAttachment,
+  WorkspaceComposerAttachment,
+} from "@/attachments/types";
+import type { AgentAttachment } from "@server/shared/messages";
+
+export function isWorkspaceAttachment(
+  attachment: ComposerAttachment | undefined,
+): attachment is WorkspaceComposerAttachment {
+  return attachment?.kind === "review" || attachment?.kind === "browser_element";
+}
+
+export function userAttachmentsOnly(
+  attachments: readonly ComposerAttachment[],
+): UserComposerAttachment[] {
+  return attachments.filter(
+    (attachment): attachment is UserComposerAttachment =>
+      attachment.kind !== "review" && attachment.kind !== "browser_element",
+  );
+}
+
+export function workspaceAttachmentToSubmitAttachment(
+  attachment: ComposerAttachment,
+): AgentAttachment | null {
+  if (attachment.kind === "browser_element") {
+    return {
+      type: "text",
+      mimeType: "text/plain",
+      title: `Browser element · ${attachment.attachment.tag}`,
+      text: attachment.attachment.formatted,
+    };
+  }
+  return attachment.kind === "review" ? attachment.attachment : null;
+}