chore: remove dead code and consolidate duplicate types (#555)

- Delete unused files: lib/ai-config.ts, components/ui/card.tsx, lib/token-counter.ts
- Remove js-tiktoken dependency (only used by deleted token-counter.ts)
- Consolidate ProviderName type: add "ollama" to model-config.ts, import in ai-providers.ts
- Consolidate DiagramOperation type: keep in chat/types.ts, import in utils.ts and hook

package.json

@@ -69,7 +69,6 @@
         "clsx": "^2.1.1",
         "cmdk": "^1.1.1",
         "idb": "^8.0.3",
-        "js-tiktoken": "^1.0.21",
         "jsonrepair": "^3.13.1",
         "lucide-react": "^0.562.0",
         "motion": "^12.23.25",