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>

frontend/src/components/modal/VideoCanvas.vue

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+<template>
+  <div class="fc-canvas fc-canvas--video">
+    <video
+      v-if="playable" :src="src" controls playsinline preload="metadata"
+      class="fc-canvas__video"
+    />
+    <div v-else class="fc-canvas__unsupported">
+      <v-icon size="56" icon="mdi-video-off-outline" />
+      <h3 class="fc-canvas__unsupported-title">Format not browser-playable</h3>
+      <p>
+        This video isn't in a format the browser can play natively
+        (<code>{{ mime }}</code>). Transcoding to MP4 lands in FC-2e —
+        until then, you can download the original.
+      </p>
+      <v-btn :href="src" download variant="tonal" rounded="pill">
+        <v-icon start>mdi-download</v-icon>
+        Download original
+      </v-btn>
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { computed } from 'vue'
+
+const props = defineProps({ src: String, mime: String })
+
+const BROWSER_PLAYABLE = new Set([
+  'video/mp4', 'video/webm', 'video/ogg', 'video/quicktime'
+])
+const playable = computed(() => BROWSER_PLAYABLE.has(props.mime))
+</script>
+
+<style scoped>
+.fc-canvas {
+  flex: 1;
+  background: rgb(var(--v-theme-background));
+  display: flex; align-items: center; justify-content: center;
+}
+.fc-canvas__video { max-width: 100%; max-height: 100%; }
+.fc-canvas__unsupported {
+  text-align: center;
+  max-width: 520px;
+  padding: 32px;
+  color: rgb(var(--v-theme-on-surface));
+}
+.fc-canvas__unsupported-title {
+  font-family: 'Fraunces', Georgia, serif;
+  font-size: 22px;
+  font-weight: 500;
+  margin: 12px 0 8px;
+}
+code {
+  background: rgb(var(--v-theme-surface-light));
+  padding: 1px 6px;
+  border-radius: 4px;
+  font-family: 'JetBrains Mono', monospace;
+}
+</style>