WebApp: Cast FormInputNumber value to Number onInput

Fix #104432, where the form throws an error and cannot be saved. It is
happening because the backend is expecting `number`, not `string`, and
whenever the user types, the value in the input gets changed to a
string. The solution is to cast `FormInputNumber` value to a number
`onInput` to always maintain the `number` type.

Pull Request: https://projects.blender.org/studio/flamenco/pulls/104434

web/app/src/components/settings/FormInputNumber.vue

@@ -67,7 +67,7 @@ export default {
   methods: {
     onInput(event) {
       // Update the v-model value
-      this.$emit('update:value', event.target.value);
+      this.$emit('update:value', Number(event.target.value));
     },
     onChange(event) {
       // Supports .lazy