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/helpers/db.ts

@@ -11,7 +11,7 @@ export function createTestDb() {
     CREATE TABLE categories (
       id INTEGER PRIMARY KEY AUTOINCREMENT,
       name TEXT NOT NULL UNIQUE,
-      emoji TEXT NOT NULL DEFAULT '📦',
+      icon TEXT NOT NULL DEFAULT 'package',
       created_at INTEGER NOT NULL DEFAULT (unixepoch())
     )
   `);
@@ -87,7 +87,7 @@ export function createTestDb() {
 
   // Seed default Uncategorized category
   db.insert(schema.categories)
-    .values({ name: "Uncategorized", emoji: "\u{1F4E6}" })
+    .values({ name: "Uncategorized", icon: "package" })
     .run();
 
   return db;