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 } },
     ))
   }