feat(native): TREK Cloud subscriptions through the App Store

The plan picker sent buyers to the website, which the App Store does not allow.
It now sells through StoreKit 2 in the TrekShell plugin (feature storeKit):
App Store price strings in the buyer's currency, the purchase sheet, the signed
transaction redeemed at the control plane, and the transaction finished only
once the instance exists. A purchase whose redeem failed is redeemed again on
retry, never bought twice.

Restore purchases, the auto-renewal terms and the links to the terms of use and
the privacy policy sit under the plans. Where no store purchase is possible
(Android, an older binary) the picker shows the plans with no way to buy and no
pointer to the web.

client/native-shell/shell.css

@@ -881,6 +881,20 @@ a.linkbtn {
   margin: 2px 0 0;
   line-height: 1.45;
 }
+/* Under the plans: restore, the renewal terms, the legal links. */
+.plans-foot {
+  display: flex;
+  flex-direction: column;
+  gap: 10px;
+  margin-top: 4px;
+}
+.plans-foot .hint {
+  text-align: center;
+  margin: 0;
+}
+.plans-foot .hint:empty {
+  display: none;
+}
 
 /* ── Claim: address + email, and the provisioning wait ───────────────────── */
 /* One rounded field holding the address input and its fixed ".liketrek.com"