feat(loyalty): transaction categories — admin UI + web terminal

Admin merchant detail page:
- New "Transaction Categories" section with store selector
- Inline add form, activate/deactivate toggle, delete button
- Categories CRUD via /admin/loyalty/stores/{id}/categories API

Web terminal:
- Loads categories on init via /store/loyalty/categories
- Category pill selector shown in PIN modal before stamp/earn actions
- Selected category_id passed to stamp and points API calls
- Categories are optional (selector hidden when none configured)

4 new i18n keys (EN).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/locales/en.json

@@ -480,6 +480,9 @@
             "table_points_earned": "Points Earned",
             "table_points_redeemed": "Points Redeemed",
             "table_transactions_30d": "Transactions (30d)",
+            "transaction_categories": "Transaction Categories",
+            "select_store": "Select a store...",
+            "no_categories": "No categories configured. Click Add to create one.",
             "admin_policy_settings": "Admin Policy Settings",
             "staff_pin_policy": "Staff PIN Policy",
             "self_enrollment": "Self Enrollment",
@@ -709,6 +712,7 @@
             "reward_redeemed": "Reward redeemed: {name}",
             "card_label": "Card",
             "confirm": "Confirm",
+            "select_category": "Category (what was sold)",
             "pin_authorize_text": "Enter your staff PIN to authorize this transaction",
             "free_item": "Free item",
             "reward_label": "Reward",