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/schema.ts

@@ -3,7 +3,7 @@ import { sqliteTable, text, integer, real } from "drizzle-orm/sqlite-core";
 export const categories = sqliteTable("categories", {
   id: integer("id").primaryKey({ autoIncrement: true }),
   name: text("name").notNull().unique(),
-  emoji: text("emoji").notNull().default("\u{1F4E6}"),
+  icon: text("icon").notNull().default("package"),
   createdAt: integer("created_at", { mode: "timestamp" })
     .notNull()
     .$defaultFn(() => new Date()),