feat(fc2a): add ImageCanvas (pan/zoom) and VideoCanvas (with format gate)

ImageCanvas mounts an <img> wrapped in the usePanZoom composable —
spreading its handlers onto the container element. Zoom resets when src
changes so prev/next nav doesn't carry zoom state into the next image.

VideoCanvas plays MP4/WebM/Ogg/MOV natively. Non-playable formats render
a friendly explainer with a download fallback, naming the actual MIME so
operators know what they're dealing with, and pointing forward at FC-2e
where transcoding lands.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/modal/ImageCanvas.vue

@@ -0,0 +1,50 @@
+<template>
+  <div
+    class="fc-canvas"
+    :class="{ 'fc-canvas--zoomed': panZoom.state.scale > 1 }"
+    @wheel="panZoom.handlers.onWheel"
+    @pointerdown="panZoom.handlers.onPointerDown"
+    @pointermove="panZoom.handlers.onPointerMove"
+    @pointerup="panZoom.handlers.onPointerUp"
+    @click="panZoom.handlers.onClick"
+  >
+    <img
+      :src="src" :alt="alt"
+      :style="{
+        transform: `translate(${panZoom.state.x}px, ${panZoom.state.y}px) scale(${panZoom.state.scale})`
+      }"
+      draggable="false"
+    >
+  </div>
+</template>
+
+<script setup>
+import { watch } from 'vue'
+import { usePanZoom } from '../../composables/usePanZoom.js'
+
+const props = defineProps({ src: String, alt: String })
+const panZoom = usePanZoom()
+
+// Reset zoom when the src changes (prev/next nav).
+watch(() => props.src, () => panZoom.reset())
+</script>
+
+<style scoped>
+.fc-canvas {
+  flex: 1;
+  position: relative;
+  background: rgb(var(--v-theme-background));
+  overflow: hidden;
+  cursor: zoom-in;
+  user-select: none;
+  display: flex; align-items: center; justify-content: center;
+}
+.fc-canvas--zoomed { cursor: grab; }
+.fc-canvas--zoomed:active { cursor: grabbing; }
+.fc-canvas img {
+  max-width: 100%; max-height: 100%;
+  transform-origin: center;
+  transition: transform 100ms ease-out;
+  pointer-events: none;
+}
+</style>