fifa17 store: make My Packs reward tiles openable (free coins row)

Reward/My-Packs tiles were emitted with no currencies row (dropped to avoid an
"undefined" payment label). But FIFA 17 opens My Packs through the store PURCHASE
flow (My Packs is a client-side filter over the store catalogue; the open-vs-buy
fork is client-side and no response selects it), so a tile with no purchase path
is not actionable — clicking it navigates instead of opening, sending no request.

Fix: keep the coins currency at the pack price (0 for reward packs; no extPrice,
so no `or %1s` mtx bug). The client then treats the tile as free and clicking
sends POST /purchased, which the server opens for free (owned-only redeem, no
debit). The empty-My-Packs sentinel keeps no currency row so it stays
non-openable. pack70 golden regenerated.

LIVE-PROVEN 2026-08-18: a reward Silver Pack opened and revealed cards on the
retail client (host log: POST /purchased/items 200 -> GET /purchased/items).

openfut-adapter-fifa17/tests/fixtures/purchasegroup_pack70.json

@@ -212,6 +212,13 @@
       "isPremium": false,
       "sortPriority": 1,
       "displayGroupAssetId": 3,
+      "currencies": [
+        {
+          "name": "coins",
+          "funds": 0,
+          "finalFunds": 0
+        }
+      ],
       "packContentInfo": {
         "bronzeQuantity": 0,
         "silverQuantity": 0,