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/ProseImg.vue

@@ -0,0 +1,24 @@
+<script setup lang="ts">
+interface Props {
+  src: string
+  alt?: string
+  title?: string
+  width?: string | number
+  height?: string | number
+}
+const props = withDefaults(defineProps<Props>(), {
+  alt: '',
+})
+</script>
+
+<template>
+  <NuxtImg
+    :src="props.src"
+    :alt="props.alt"
+    :title="props.title"
+    :width="props.width"
+    :height="props.height"
+    class="rounded-lg w-full"
+    sizes="sm:600px md:800px lg:1000px"
+  />
+</template>