feat(audit): undo button for merges, hide manual games

The audit log shows Undo on a merge row for 30 days, with a confirm
dialog and clear errors. The details page follows a merged key to the
kept game. Hide and Show now work for manual games too. de/en texts.

public/i18n/en.json

@@ -269,7 +269,7 @@
     "OPEN_IN_STEAM": "View on Steam",
     "HIDE": "Hide game",
     "UNHIDE": "Show game again",
-    "HIDDEN_NOTE": "Hidden: this game is not in your library lists, stats or public profile. Only its playtime is still updated.",
+    "HIDDEN_NOTE": "Hidden: this game is not in your library lists, stats or public profile.",
     "HIDDEN_TOAST": "Game hidden",
     "SHOWN_TOAST": "Game shown again",
     "HIDE_ERROR": "Could not change the game.",
@@ -650,7 +650,7 @@
     "OPEN": "Merge duplicate",
     "TITLE": "Merge with the same game",
     "HINT": "Find the other entry of \"{{name}}\", e.g. the Steam game or a game added by hand. The merge applies to every user.",
-    "CONFIRM": "Merge \"{{name}}\" and \"{{other}}\" into one game for every user? Libraries with both keep one entry with the status, rating and notes set last. This cannot be undone.",
+    "CONFIRM": "Merge \"{{name}}\" and \"{{other}}\" into one game for every user? Libraries with both keep one entry with the status, rating and notes set last. This can be undone for 30 days in the audit log.",
     "CONFIRM_BUTTON": "Merge",
     "BACK": "Back",
     "DONE": "Games merged",
@@ -710,11 +710,24 @@
     "NO_ACCESS": "Only auditors can open this page.",
     "LOAD_ERROR": "The audit log could not be loaded.",
     "USER_ID": "user {{id}}",
+    "UNDO": {
+      "BUTTON": "Undo",
+      "CONFIRM": "Undo the merge of \"{{name}}\" and \"{{other}}\"? Both games come back for every user. Status, rating and notes set since the merge are kept.",
+      "DONE": "Merge undone.",
+      "ERROR": "The merge could not be undone.",
+      "ERROR_CODES": {
+        "MERGE_NOT_FOUND": "This merge does not exist.",
+        "UNDO_NOT_AVAILABLE": "This merge was already undone or is too old to undo.",
+        "NEWER_MERGE": "Undo the newer merge of this game first.",
+        "UNDO_EXPIRED": "The undo of this merge expired after 30 days."
+      }
+    },
     "PAGE_OF": "Page {{current}} of {{total}}",
     "ACTION": {
       "LABEL": "Action",
       "ALL": "All actions",
       "GAME_MERGE": "Merge",
+      "GAME_MERGE_UNDO": "Merge undone",
       "GAME_LINK": "Link",
       "GAME_UNLINK": "Unlink",
       "GAME_LINK_DISMISS": "Dismissed",