refactor: align tariff builder with bot functionality

- Remove unused fields: custom_days, custom_traffic (bot doesn't use them)
- Add promo groups selection to tariff form
- Add API endpoint for fetching available promo groups
- Make traffic topup packages dynamic (add/remove any number)
- Fix stuck 0 in number inputs when clearing
- Add validation messages explaining why save button is blocked
- Match bot validation: name 2-50 chars, device_limit >= 1, tier_level 1-10
- Add missing i18n translations for new fields and validation errors

src/locales/en.json

@@ -1251,6 +1251,7 @@
       "nameLabel": "Tariff name",
       "nameExamplePeriod": "E.g.: Standard",
       "nameExampleDaily": "E.g.: Daily",
+      "nameHint": "2 to 50 characters",
       "descriptionLabel": "Description",
       "descriptionPlaceholder2": "Brief tariff description",
       "trafficLimitLabel": "Traffic limit",
@@ -1267,8 +1268,12 @@
       "addButton": "Add",
       "noPeriodsHint": "No added periods. Add at least one period.",
       "daysShort": "d.",
+      "serversTitle": "Servers",
       "serversTabHint": "Select servers available on this tariff.",
       "noServersAvailable": "No servers available",
+      "promoGroupsTitle": "Promo Groups",
+      "promoGroupsHint": "Select promo groups that have access to this tariff. If none selected — available to everyone.",
+      "noPromoGroups": "No promo groups",
       "extraDeviceTitle": "Additional devices",
       "devicePriceLabel": "Device price (30 days):",
       "devicePriceHint": "0 = device purchase unavailable",
@@ -1278,6 +1283,8 @@
       "trafficMaxLimitLabel": "Max. limit:",
       "trafficLimitHint2": "(0 = no limits)",
       "trafficPackagesLabel": "Traffic packages:",
+      "addPackageTitle": "Add package",
+      "noPackagesHint": "No packages added. Add at least one package.",
       "gbPackageUnit": "GB:",
       "customDaysTitle": "Custom number of days",
       "customDaysDesc": "User selects subscription duration",
@@ -1309,7 +1316,18 @@
       "dailyPriceLabel": "Price per day",
       "currencyPerDay": "\u20bd/day",
       "dailyDeductionDesc": "Charged daily from user's balance",
-      "dailyPriceRequired": "Enter a price per day greater than 0"
+      "dailyPriceRequired": "Enter a price per day greater than 0",
+      "cannotSave": "Cannot save tariff:",
+      "validation": {
+        "nameRequired": "Enter tariff name",
+        "nameLength": "Name must be 2 to 50 characters",
+        "deviceLimitRequired": "Specify number of devices (minimum 1)",
+        "tierLevelInvalid": "Tier level must be between 1 and 10",
+        "fieldsRequired": "Fill in required fields (devices, tier level)",
+        "periodsRequired": "Add at least one period",
+        "dailyPriceRequired": "Enter a price per day greater than 0",
+        "trafficPackagesRequired": "Add at least one traffic package (or disable traffic topup)"
+      }
     },
     "servers": {
       "title": "Server Management",