feat(posts): support short titles for resource cards

Add optional post-level and localized title fields, and use a new
postTitleText helper for Resource.title so card/list surfaces prefer short
backend-provided titles instead of full body text. When title is missing,
fallback to the first non-empty body line, then filename, then post id.

Document the backend handoff in docs/posts-title-api.md alongside the other
backend task docs.

src/utils/postResourceAdapter.ts

@@ -1,6 +1,9 @@
 import type { Category, Resource } from "../api";
 import type { Attachment, Post } from "../types/post";
-import { postDisplayText } from "../components/messageStream/utils/postText";
+import {
+  postDisplayText,
+  postTitleText,
+} from "../components/messageStream/utils/postText";
 
 export type PostBackedResource = Resource & {
   downloadPostId?: string;
@@ -35,7 +38,7 @@ export function postToResource(
   categories: Category[] = [],
 ): PostBackedResource {
   const first = post.attachments[0];
-  const title = postDisplayText(post, lang) || first?.filename || post.id;
+  const title = postTitleText(post, lang) || first?.filename || post.id;
   const category = categories.find((c) => c.id === post.categoryId);
   return {
     id: post.id,