feat(play-next): show why each game was picked

Each card shows a reason line under the game name. Issue #28.

public/i18n/en.json

@@ -409,7 +409,12 @@
   },
   "PLAY_NEXT": {
     "TITLE": "What to play next",
-    "SUBTITLE": "Three games from your backlog, high priority first. Pick a length and a mood.",
+    "SUBTITLE": "Three games from your backlog: one like a game you played a lot, the one waiting longest, and one at random. Pick a length and a mood.",
+    "REASON": {
+      "SIMILAR": "Because you played {{name}} ({{playtime}})",
+      "LONGEST": "On your list since {{date}}",
+      "RANDOM": "Random pick"
+    },
     "BUDGET_LABEL": "Time to beat",
     "BUDGET": {
       "SHORT": "Up to 5 h",