refactor: convert GiftDetailsStep update handlers from emits to function props

- Replace @Emit("update:description"), @Emit("update:amount"), @Emit("update:unitCode") with function props
- Add onUpdateDescription, onUpdateAmount, onUpdateUnitCode function props with TypeScript typing
- Update GiftedDialog to use new function prop interface for update handlers
- Remove emit decorators and methods for update events
- Keep emit pattern for non-update events (edit-entity, explain-data, submit, cancel)
- Improve component documentation to reflect new architecture

This change provides better parent control over validation and update behavior
for form fields while maintaining existing functionality for other events.

src/components/GiftedDialog.vue

@@ -39,9 +39,9 @@
         :offer-id="offerId"
         :from-project-id="fromProjectId"
         :to-project-id="toProjectId"
-        @update:description="description = $event"
-        @update:amount="handleAmountUpdate"
-        @update:unit-code="unitCode = $event"
+        :on-update-description="(desc: string) => (description = desc)"
+        :on-update-amount="handleAmountUpdate"
+        :on-update-unit-code="(code: string) => (unitCode = code)"
         @edit-entity="handleEditEntity"
         @explain-data="explainData"
         @submit="handleSubmit"