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>

src/db/seed.ts

@@ -7,7 +7,7 @@ export function seedDefaults() {
     db.insert(categories)
       .values({
         name: "Uncategorized",
-        emoji: "\u{1F4E6}",
+        icon: "package",
       })
       .run();
   }