fix(ui): align usage data contracts, add timezone setting, and fix empty usage page (#146)

- Fix 6 data contract mismatches between Go backend JSON tags and React
  frontend TypeScript interfaces (field renames, response envelope changes)
- Add timezone selector to topbar with 12 common timezone options
- Replace date-fns formatting with native Intl.DateTimeFormat for
  timezone-aware chart labels (reduces bundle ~20KB)
- Add missing SnapshotTimeSeries fields (memory_docs, memory_chunks,
  kg_entities, kg_relations) that caused empty usage page
- Add error banner to usage page for API error visibility
- Sanitize backend error messages in usage HTTP handlers
- Add batch chunking (max 3000 rows) for snapshot upserts
- Remove userId display from topbar
- Add usage analytics i18n strings for en/vi/zh

ui/web/package.json

@@ -20,6 +20,7 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "d3-force": "^3.0.0",
+    "date-fns": "^4.1.0",
     "framer-motion": "^12.34.3",
     "highlight.js": "^11.11.1",
     "i18next": "^25.8.15",
@@ -30,6 +31,7 @@
     "react-i18next": "^16.5.6",
     "react-markdown": "^10.1.0",
     "react-router": "^7.1.0",
+    "recharts": "^3.8.0",
     "rehype-highlight": "^7.0.2",
     "remark-gfm": "^4.0.1",
     "tailwind-merge": "^2.6.0",