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/components/messageStream/utils/postText.ts

@@ -11,3 +11,19 @@ export function postDisplayText(post: Post, lang: string): string {
     ""
   );
 }
+
+export function postTitleText(post: Post, lang: string): string {
+  const key = localizationKey(lang);
+  const localized =
+    post.localizations?.[key as keyof typeof post.localizations];
+  const explicitTitle = localized?.title?.trim() || post.title?.trim();
+  if (explicitTitle) return explicitTitle;
+
+  const text = postDisplayText(post, lang);
+  return (
+    text
+      .split(/\n{1,}/)
+      .map((line) => line.trim())
+      .find(Boolean) || ""
+  );
+}