feat(library): add not the same game button to editions

Part of Gitea #34 slice 1 of 3: one entry per game across platforms.
Status, rating and notes are shared, so the detail page reloads after a save.

README.md

@@ -57,10 +57,12 @@ To Be Played is split into three repositories:
   finished games per year of all users.
 - **Library**: status filters, source filter (Steam, PlayStation, manual; a linked set counts on each of its sources), "short games" filter (up to 10 h), search, sorting by playtime, recently seen, newest
   in library, rating or priority, paging. Each row shows the unlocked achievements.
-- **Editions**: an auditor links the editions of one game (Steam and PS5 copy, remaster, port) for every user. A user
-  who owns two or more of a linked set sees "Also in your library" on the details page, an "x2" badge in the library
-  and the filter "Hide duplicates"; the numbers count the set once. "Same game as..." on the details page asks the
-  auditors for a link.
+- **Editions**: the editions of one game (Steam and PS5 copy, remaster, port) are linked automatically from IGDB's
+  "same game" key; an auditor can also link or unlink them for every user. A user who owns two or more of a linked
+  set sees "Also in your library" on the details page, an "x2" badge in the library and the filter "Hide
+  duplicates"; the numbers count the set once. The editions of a set share one status, rating and notes (the page
+  reloads after a save to show it). "Not the same game" splits the game off its set for this user only, and it stays
+  split. "Same game as..." on the details page asks the auditors for a link.
 - **Add a game by name** for games outside Steam (PlayStation, Switch, Xbox, physical): search IGDB, pick a hit,
   a platform and optional hours. Works without a Steam ID. Games already in ToBePlayed come first. Check several
   hits to add them at once through the import review.
@@ -257,6 +259,17 @@ flowchart LR
   which question is open; `src/utils/questions.js` only loads the list and sends the answer. A yes to Finished? gets
   the Rate it? card back at once. `409` / `404` reload the list. `RatingStars` is shared with `BacklogCard`.
 
+```mermaid
+flowchart LR
+    Detail["GameDetail: EditionsCard"] -->|"Not the same game"| Split["Core DELETE /api/library/games/{appid}/set"]
+    Detail -->|"Same game as... (request)"| Request["Core POST /api/library/games/{appid}/link-requests"]
+    Detail -->|"Unlink (auditor)"| Unlink["Core DELETE /api/library/games/{appid}/link"]
+    Split --> Reload["reload the detail"]
+    Backlog["BacklogCard save"] -->|"game has editions"| Reload
+```
+- `EditionsCard` shows "Not the same game" on every game that has other editions in the own library. The call goes
+  through `src/utils/links.js` (`splitGame`); `404 NOT_LINKED` shows the generic error toast.
+
 ```mermaid
 flowchart LR
     Header["AppHeader badge"] --> Count["Core /api/library/questions/count"]