feat(app): unify composer attachments with pill + lightbox redesign

Images and GitHub issues/PRs now live in one draft-store list (ComposerAttachment union) with cwd persisted on the draft. Paperclip becomes a + drop-up menu; queue button removed. Each pill uses a shared AttachmentPill wrapper with a corner-X that mirrors the auto-update toast spec. Pill body opens the attachment: GitHub via openExternalUrl, images via a new fullscreen AttachmentLightbox. GitHub picker auto-closes on select.

packages/app/src/attachments/types.ts

@@ -1,3 +1,5 @@
+import type { GitHubSearchItem } from "@server/shared/messages";
+
 export type AttachmentStorageType = "web-indexeddb" | "desktop-file" | "native-file";
 
 export interface AttachmentMetadata {
@@ -15,6 +17,11 @@ export interface AttachmentMetadata {
   createdAt: number;
 }
 
+export type ComposerAttachment =
+  | { kind: "image"; metadata: AttachmentMetadata }
+  | { kind: "github_issue"; item: GitHubSearchItem }
+  | { kind: "github_pr"; item: GitHubSearchItem };
+
 export type AttachmentDataSource =
   | { kind: "blob"; blob: Blob }
   | { kind: "data_url"; dataUrl: string }