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/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 }
 })