fix(loyalty): drop unregistered device-tablet icon, guard QR template against null

- The icon registry has no 'device-tablet' (closest available is 'phone',
  which is already used for similar device-mobile contexts). Replace
  uses I added in the merchant menu item and the empty state.

- The pairing-QR modal uses x-show on the QR/payload blocks, but x-show
  only toggles display while Alpine still evaluates child expressions.
  pairingResult is null on first render, so the template threw "can't
  access property 'qr_png_base64'/'setup_payload', pairingResult is
  null" until pairing actually fired. Wrap the block in
  <template x-if="pairingResult"> so the bindings only mount when the
  data exists.

(There is a third 'device-tablet' reference in store/analytics.html
that predates this work — leaving it for a separate cleanup since
it's not on the Android-rollout path.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/modules/loyalty/definition.py

@@ -282,7 +282,7 @@ loyalty_module = ModuleDefinition(
                     MenuItemDefinition(
                         id="devices",
                         label_key="loyalty.menu.terminal_devices",
-                        icon="device-tablet",
+                        icon="phone",
                         route="/merchants/loyalty/devices",
                         order=32,
                     ),