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/shared/schemas.ts

@@ -16,13 +16,13 @@ export const updateItemSchema = createItemSchema.partial().extend({
 
 export const createCategorySchema = z.object({
   name: z.string().min(1, "Category name is required"),
-  emoji: z.string().min(1).max(4).default("\u{1F4E6}"),
+  icon: z.string().min(1).max(50).default("package"),
 });
 
 export const updateCategorySchema = z.object({
   id: z.number().int().positive(),
   name: z.string().min(1).optional(),
-  emoji: z.string().min(1).max(4).optional(),
+  icon: z.string().min(1).max(50).optional(),
 });
 
 // Thread schemas