feat(tagging): allowlist tuning dashboard + post-accept toast + merge preview UI (#7c/#7d/#8b)

Cluster B frontend, milestone #99.

#7c: AllowlistTable gains Applied + Covers columns and a live 'covers ~N at T'
projection as the operator drags a row's threshold (debounced coverage call,
then commits the threshold). allowlist store gains coverage(tagId, threshold)
and refreshes coverage_count after a save.

#7d: suggestions store surfaces a non-blocking toast when accept/alias newly
allowlists a tag — '<verb>: <tag> — allowlisted, auto-applying to ~N images'
(N is the projection; apply runs async). Falls back to the plain toast when
the tag was already allowlisted.

#8b: TagsView merge picker now previews the merge via usePreviewCommit before
committing — shows images moving / already-on-target / series pages / alias-or-
delete / a thumbnail sample, blocks the Merge button on an incompatible
kind/fandom. adminStore.mergeTags gains a dryRun option.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCUHUGQLrBrkgyk1t49kpX

frontend/src/stores/admin.js

@@ -65,10 +65,11 @@ export const useAdminStore = defineStore('admin', () => {
     return _guard(() => api.delete(`/api/admin/tags/${tagId}`))
   }
 
-  function mergeTags(destId, sourceId) {
+  // dryRun → {preview: {...}} (non-mutating counts + sample); else {result}.
+  function mergeTags(destId, sourceId, { dryRun = false } = {}) {
     return _guard(() => api.post(
       `/api/admin/tags/${destId}/merge`,
-      { body: { source_id: sourceId } },
+      { body: { source_id: sourceId, dry_run: dryRun } },
     ))
   }
 

frontend/src/stores/allowlist.js

@@ -18,7 +18,21 @@ export const useAllowlistStore = defineStore('allowlist', () => {
       body: { min_confidence: minConfidence }
     })
     const r = rows.value.find(x => x.tag_id === tagId)
-    if (r) r.min_confidence = minConfidence
+    if (r) {
+      r.min_confidence = minConfidence
+      // The committed threshold changed the covered pool — refresh the row's
+      // coverage so the table stays truthful after a save.
+      try { r.coverage_count = (await coverage(tagId, minConfidence)).count }
+      catch { /* leave the stale count rather than blank it */ }
+    }
+  }
+
+  // Live "at threshold T, a sweep would cover ~N images" projection for the
+  // tuning dashboard. Returns { count, threshold }.
+  async function coverage(tagId, threshold) {
+    return api.get(`/api/tags/${tagId}/allowlist/coverage`, {
+      params: { threshold }
+    })
   }
 
   async function remove(tagId) {
@@ -26,5 +40,5 @@ export const useAllowlistStore = defineStore('allowlist', () => {
     rows.value = rows.value.filter(x => x.tag_id !== tagId)
   }
 
-  return { rows, loading, load, updateThreshold, remove }
+  return { rows, loading, load, updateThreshold, coverage, remove }
 })

frontend/src/stores/suggestions.js

@@ -100,7 +100,7 @@ export const useSuggestionsStore = defineStore('suggestions', () => {
       })
       tagId = created.id
     }
-    await api.post(`/api/images/${imageId}/suggestions/accept`, {
+    const res = await api.post(`/api/images/${imageId}/suggestions/accept`, {
       body: { tag_id: tagId }
     })
     // Only drop from THIS image's category list — if the user navigated,
@@ -108,10 +108,23 @@ export const useSuggestionsStore = defineStore('suggestions', () => {
     if (currentImageId === imageId) {
       _dropEverywhere(suggestion)
     }
-    toast({
-      text: `Tagged: ${suggestion.display_name}`,
-      type: 'success'
-    })
+    _acceptToast('Tagged', suggestion.display_name, res)
+  }
+
+  // One non-blocking toast for accept/alias. When the accept newly allowlisted
+  // the tag, surface the coverage PROJECTION (#7) so the operator sees the
+  // auto-apply reach without a blocking pre-accept preview — the apply itself
+  // runs async, hence "~N".
+  function _acceptToast(verb, displayName, res) {
+    if (res?.allowlisted) {
+      const n = res.projected_count
+      toast({
+        text: `${verb}: ${displayName} — allowlisted, auto-applying to ~${n} image${n === 1 ? '' : 's'}`,
+        type: 'success'
+      })
+    } else {
+      toast({ text: `${verb}: ${displayName}`, type: 'success' })
+    }
   }
 
   async function aliasAccept(suggestion, canonicalTagId) {
@@ -123,7 +136,7 @@ export const useSuggestionsStore = defineStore('suggestions', () => {
     // reappearing unaliased. raw_name is null only for centroid hits, which
     // can't be aliased (the UI hides the action for them).
     const aliasString = suggestion.raw_name ?? suggestion.display_name
-    await api.post(`/api/images/${imageId}/suggestions/alias`, {
+    const res = await api.post(`/api/images/${imageId}/suggestions/alias`, {
       body: {
         alias_string: aliasString,
         alias_category: suggestion.category,
@@ -133,10 +146,7 @@ export const useSuggestionsStore = defineStore('suggestions', () => {
     if (currentImageId === imageId) {
       _dropEverywhere(suggestion)
     }
-    toast({
-      text: `Aliased & tagged: ${suggestion.display_name}`,
-      type: 'success'
-    })
+    _acceptToast('Aliased & tagged', suggestion.display_name, res)
   }
 
   // Remove the alias behind an aliased suggestion (the raw prediction reverts to