feat(dashboard): playtime chart, recently played and a playing hint

The dashboard shows the playtime of the last 30 days and Steam's recently
played games. The detail page offers to set a backlog game to Playing when
Steam reports recent playtime; unsaved edits in the backlog card stay.

src/utils/backlog.js

@@ -13,6 +13,18 @@ export const toBacklogRequest = ({status, rating, notes, priority}) => ({
     priority: priority === NO_PRIORITY ? null : priority,
 });
 
+/**
+ * PUT /api/library/games/{appid}/backlog. The caller handles the response status.
+ *
+ * @param entry a backlog entry in request form, see toBacklogRequest
+ */
+export const saveBacklogEntry = (appid, entry) =>
+    fetch(`${import.meta.env.VITE_CORE_API_URL}/api/library/games/${appid}/backlog`, {
+        method: 'PUT',
+        headers: {'Authorization': `Bearer ${localStorage.getItem('tbp_jwt')}`, 'Content-Type': 'application/json'},
+        body: JSON.stringify(entry),
+    });
+
 export const libraryQuery = ({page, size, sort, status, q}) => {
     const params = new URLSearchParams({page: String(page), size: String(size), sort});
     if (status) params.set('status', status);