refactor: implement i18n in TierSelectorEnhanced (phase 3)

Refactored TierSelectorEnhanced to use react-i18next translations:
- Tier selection UI labels (selected/select)
- Popular badge
- Pricing labels
- Comparison table headers and rows
- Delivery time labels

Added translation keys to es.json and en.json:
- tierSelector.popular, selected, select, oneTime

Build verified successfully.

https://claude.ai/code/session_4f888c33-8937-4db8-8a9d-ddc9ac51a725

frontend/locales/en.json

@@ -80,6 +80,10 @@
     "viewResults": "View Results"
   },
   "tierSelector": {
+    "popular": "POPULAR",
+    "selected": "Selected",
+    "select": "Select",
+    "oneTime": "one-time",
     "hideComparison": "Hide Comparison",
     "viewComparison": "View Detailed Comparison",
     "comparison": "Tier Comparison",