feat(ui): hover a suggestion → highlight the crop region it came from (#133 step 3)

The payoff: hover a suggestion in the rail and the exact crop that produced it
lights up on the image (a booru:head, a panel, a figure); a null-grounding tag
shows a subtle dashed whole-image frame ('global vector won, not a crop').
ImageCanvas gains a grounding overlay that tracks the <img>'s live bounding rect
(correct under object-fit letterboxing + pan/zoom) and draws the normalized bbox
+ a detector/kind label. SuggestionItem sets the hovered grounding via
provide/inject (no 4-level event relay through TagPanel/SuggestionsPanel/group);
ImageViewer AND ExploreView provide it + pass it to their canvas. Overlay is
pointer-events:none so it never blocks pan/zoom/click. Videos out of v1 scope.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/views/ExploreView.vue

@@ -98,6 +98,7 @@
                 :key="store.anchor.id"
                 :src="store.anchor.image_url"
                 :alt="`Image ${store.anchor.id}`"
+                :hover="hoverGrounding"
               />
               <VideoCanvas
                 v-else
@@ -131,7 +132,7 @@
 </template>
 
 <script setup>
-import { computed, nextTick, onMounted, onUnmounted, ref, watch } from 'vue'
+import { computed, nextTick, onMounted, onUnmounted, provide, ref, watch } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { useApi } from '../composables/useApi.js'
 import { useExploreStore } from '../stores/explore.js'
@@ -152,6 +153,11 @@ const modal = useModalStore()
 
 const anchorId = computed(() => route.params.imageId || null)
 const isVideo = computed(() => !!store.anchor?.mime?.startsWith('video/'))
+
+// #1206: hovering a suggestion in the rail highlights the crop it came from on
+// the anchor image (same provide/inject as the modal viewer).
+const hoverGrounding = ref(null)
+provide('fcSuggestionHover', hoverGrounding)
 const seeding = ref(false)
 const seedError = ref(null)
 const tagPanelRef = ref(null)