feat(library): show playtime per platform on game detail

Hero and library card show the set total. Fall back to the
row's own minutes when the API sends no total.

README.md

@@ -65,6 +65,10 @@ To Be Played is split into three repositories:
   once (no "Hide duplicates" filter any more), and the numbers count it 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.
+- **Detail per platform**: the details page of a game with several platforms (or a PlayStation title on a Steam game)
+  shows `PlatformsCard` above the editions: one row per platform with playtime, last played and the achievement or
+  trophy progress of that platform, a total row, and a link to the edition whose page lists the achievements. The hero
+  playtime and the library card show the set total (`totalPlaytimeMinutes`); sorting and the stats stay per edition.
 - **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.
@@ -268,6 +272,13 @@ flowchart LR
     Row -.->|"no platforms (hidden game)"| Badge["single platform badge"]
 ```
 
+```mermaid
+flowchart LR
+    DetailData["Core GET /api/library/games/{appid}: platforms[] + totalPlaytimeMinutes"] --> Platforms["PlatformsCard: row per platform, total"]
+    DetailData --> Hero["GameHeroCard: total playtime"]
+    Platforms -->|"row of another edition"| Other["/library/{appid} of that edition"]
+```
+
 ```mermaid
 flowchart LR
     Detail["GameDetail: EditionsCard"] -->|"Not the same game"| Split["Core DELETE /api/library/games/{appid}/set"]