feat(06-01): migrate categories from emoji to Lucide icon field

- Rename emoji column to icon in schema, Zod schemas, and all services
- Add Drizzle migration with emoji-to-icon data conversion
- Update test helper, seed, and all test files for icon field
- All 87 tests pass with new icon-based schema

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

tests/services/thread.service.test.ts

@@ -100,7 +100,7 @@ describe("Thread Service", () => {
       expect(result!.candidates).toHaveLength(1);
       expect(result!.candidates[0].name).toBe("Tent A");
       expect(result!.candidates[0].categoryName).toBe("Uncategorized");
-      expect(result!.candidates[0].categoryEmoji).toBeDefined();
+      expect(result!.candidates[0].categoryIcon).toBeDefined();
     });
 
     it("returns null for non-existent thread", () => {