api: fix unread detection broken by substring match

The check `.contains("user/-/state/com.google/read")` was a substring
match, which also matched "user/-/state/com.google/reading-list" — a
category present on every article fetched from the reading list. This
caused all articles to be treated as read, so nothing ever appeared
bold in the sidebar.

Fix by using == for exact string comparison.

src/api.rs

@@ -148,7 +148,7 @@ impl Api {
         Ok(stream.items.into_iter().map(|item| {
             let unread = !item.categories.as_deref().unwrap_or_default()
                 .iter()
-                .any(|c| c.contains("user/-/state/com.google/read"));
+                .any(|c| c == "user/-/state/com.google/read");
             let content = item.summary
                 .as_ref()
                 .and_then(|s| s.content.clone())