feat: share post to other apps

lib/social.ts

@@ -62,6 +62,11 @@ export type Post = {
   author_name?: string;
   author_handle?: string;
   author_avatar_url?: string | null;
+  /** True when the calling viewer has reshared this post. Populated by
+   *  endpoints that go through OptionalAuth (single post, profile feed,
+   *  collection contents). Always false / absent for anonymous viewers.
+   *  Drives the post-card "Reshare ↔ Send" swap. */
+  viewer_has_reshared?: boolean;
 };
 
 export type Bookmark = {
@@ -130,6 +135,17 @@ export const posts = {
   mine() {
     return call<Post[]>("/posts/mine");
   },
+  /** Record a reshare. Idempotent — calling twice is a no-op server
+   *  side. Resharing your own post is rejected with 400 (not currently
+   *  surfaced as a distinct error type because the UI only ever offers
+   *  Reshare on non-owned posts). */
+  reshare(id: string) {
+    return call<void>(`/posts/${id}/reshare`, { method: "POST" });
+  },
+  /** Remove a previously recorded reshare. Idempotent. */
+  unreshare(id: string) {
+    return call<void>(`/posts/${id}/reshare`, { method: "DELETE" });
+  },
   /** R2 presigned PUT for a single post image. Returns the public URL the post should persist. */
   async uploadImage(file: File): Promise<string> {
     const { upload_url, public_url } = await call<{