feat(05-02): create MDC components ProseImg.vue and Alert.vue

- ProseImg.vue: transparent NuxtImg override for markdown images (BLOG-05)
- Alert.vue: MDC callout component with 4 types (info/warning/tip/danger) via UAlert
- ContentSlot required for MDC slot content rendering (Pitfall 4)

app/components/content/Alert.vue

@@ -0,0 +1,34 @@
+<script setup lang="ts">
+interface Props {
+  type?: 'info' | 'warning' | 'tip' | 'danger'
+}
+const props = withDefaults(defineProps<Props>(), {
+  type: 'info',
+})
+
+const iconMap = {
+  info: 'i-heroicons-information-circle',
+  warning: 'i-heroicons-exclamation-triangle',
+  tip: 'i-heroicons-light-bulb',
+  danger: 'i-heroicons-x-circle',
+}
+const colorMap = {
+  info: 'info',
+  warning: 'warning',
+  tip: 'success',
+  danger: 'error',
+}
+</script>
+
+<template>
+  <UAlert
+    :icon="iconMap[props.type]"
+    :color="colorMap[props.type] as any"
+    variant="soft"
+    class="my-4"
+  >
+    <template #description>
+      <ContentSlot :use="$slots.default" unwrap="p" />
+    </template>
+  </UAlert>
+</template>