feat: add column span support and FormRelayGrid component

[RobertBoes]

Summary

• Adds columns to FormSchema and columnSpan to FormField in @formrelay/core for grid-based form layouts
• New <FormRelayGrid> component in @formrelay/vue and @formrelay/nuxt that handles CSS grid rendering automatically via a #field scoped slot
• Depends on API changes in FormRelay/formrelay#223

Changes

@formrelay/core

• FormSchema gains columns: number (grid column count from API)
• FormField gains columnSpan: number (field width in grid columns)
• Snake-to-camel transformation for column_span → columnSpan

@formrelay/vue

• useFormRelay exposes columns computed property (defaults to 2)
• FormRelay component passes columns as a slot prop
• New <FormRelayGrid> component with #field scoped slot, columns/fields props, and optional tag prop

@formrelay/nuxt

• FormRelayGrid registered as auto-imported component

Usage

<FormRelay form-id="abc" public-key="pk_test" v-slot="{ columns, fields, values }">
  <FormRelayGrid :columns="columns" :fields="fields">
    <template #field="{ field }">
      <input :name="field.name" v-model="values[field.name]" />
    </template>
  </FormRelayGrid>
</FormRelay>

Test plan

• Core: all 74 tests pass (including updated schema transformation tests)
• Vue: all 40 tests pass (including 6 new FormRelayGrid tests)
• Verify integration with API once FormRelay/formrelay#223 is merged