feat(ui): hover an applied tag chip → highlight its grounding crop (#133 step 4)

Applied tags aren't scored live, so compute the grounding on demand: run the
tag's head over the image's max-over-bag (whole-image + concept crops), argmax
→ the region that best explains the tag on this image, mirroring what
score_image records for live suggestions.

- heads.py: extract _image_bag (now shared by score_image) + ground_applied_tag.
  Returns (grounding, has_head): has_head False = no head to localize with →
  no overlay; grounding None = the whole-image vector won → whole-image frame.
- tags.py: GET /api/images/<id>/tags/<id>/grounding → {grounding, has_head}.
- TagChip/TagPanel: applied chips inject fcSuggestionHover and fetch grounding
  on hover (cached per image+tag, race-guarded), reusing Step 3's overlay in
  both the modal and Explore. No new frontend overlay code.

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

frontend/src/components/modal/TagChip.vue

@@ -1,5 +1,5 @@
 <template>
-  <span class="fc-tag-chip">
+  <span class="fc-tag-chip" @mouseenter="onEnter" @mouseleave="onLeave">
     <v-chip
       size="small" closable
       :color="store.colorFor(tag.kind)" variant="tonal"
@@ -38,14 +38,57 @@
 </template>
 
 <script setup>
-import { computed } from 'vue'
+import { computed, inject } from 'vue'
 import { useTagStore } from '../../stores/tags.js'
+import { useApi } from '../../composables/useApi.js'
 import KebabMenu from '../common/KebabMenu.vue'
 
-const props = defineProps({ tag: { type: Object, required: true } })
+const props = defineProps({
+  tag: { type: Object, required: true },
+  // When set (the tagging panels), hovering the chip asks the backend which crop
+  // region best explains this applied tag and lights it up on the image — the
+  // same overlay the suggestion rail uses (#1206 Step 4). Omitted elsewhere →
+  // the hover is inert (no injected target, or no image to ground against).
+  imageId: { type: Number, default: null },
+})
 defineEmits(['remove', 'rename', 'set-fandom', 'navigate'])
 
 const store = useTagStore()
+const api = useApi()
+
+// #1206 Step 4: applied-tag grounding. `fcSuggestionHover` is provided by the
+// image viewer / Explore host (a no-op elsewhere). Applied tags aren't scored
+// live, so we fetch the winning region on demand and cache it per (image, tag).
+const hover = inject('fcSuggestionHover', null)
+const _groundingCache = new Map()
+// Bumped on every enter/leave so a slow fetch that resolves after the pointer
+// has moved on can't draw a stale overlay.
+let hoverSeq = 0
+
+async function onEnter () {
+  if (!hover || props.imageId == null) return
+  const seq = ++hoverSeq
+  const key = `${props.imageId}:${props.tag.id}`
+  let res = _groundingCache.get(key)
+  if (res === undefined) {
+    try {
+      res = await api.get(
+        `/api/images/${props.imageId}/tags/${props.tag.id}/grounding`
+      )
+      _groundingCache.set(key, res)
+    } catch {
+      return   // best-effort — leave the overlay untouched on error
+    }
+  }
+  if (seq !== hoverSeq) return   // pointer already left / moved to another chip
+  // No head → nothing to localize; don't draw an overlay at all. With a head,
+  // null grounding still draws the whole-image frame ("the global vector won").
+  if (res.has_head) hover.value = { g: res.grounding ?? null }
+}
+function onLeave () {
+  hoverSeq++
+  if (hover) hover.value = null
+}
 
 // Show a character's fandom inline (truncated). Falls back to a bare arrow when
 // only fandom_id is known but the name wasn't resolved (older payloads).

frontend/src/components/modal/TagPanel.vue

@@ -4,7 +4,7 @@
     <div class="fc-tag-panel__chips">
       <TagChip
         v-for="tag in host.current?.tags || []"
-        :key="tag.id" :tag="tag"
+        :key="tag.id" :tag="tag" :image-id="host.currentImageId"
         @remove="onRemove" @rename="openRename" @set-fandom="openSetFandom"
         @navigate="onNavigate"
       />