feat(library): add the merge duplicate dialog

README.md

@@ -32,7 +32,7 @@ To Be Played is split into three repositories:
 - **Library**: status filters, "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.
 - **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.
+  a platform and optional hours. Works without a Steam ID. Games already in ToBePlayed come first.
 - **Game detail**: status, priority, 1–5 star rating, notes, achievements, time to beat (story / extras / 100 %),
   a link to the game's speedrun.com page when it has verified runs, a hint when Steam shows recent playtime for a
   game still in the backlog, and one to set the status to 100 % when
@@ -62,6 +62,8 @@ To Be Played is split into three repositories:
   "Accept" hint stays until "OK". Admins write, preview and end hints on `/admin/hints` (Auth stores them).
 - **Correct game data** (AUDITOR or higher): "Correct data" on the game detail page changes name, header image,
   speedrun link, moods, endless flag and genres for every user; each field goes back to the source on its own.
+  "Merge duplicate" joins two entries of the same game (e.g. the Steam game and the same game added by hand) for
+  every user.
 - **Delete my account** in the settings: a dialog asks for the own email and the password. Auth blocks the
   account at once and removes it after 30 days; the page ends the session and goes to the landing page.
 - **Build version** in the settings: commit and build time of Frontend, Auth and Core, each hash links to its