Online browser + post detail: SPA talks to e621 directly

Backend:
- POST /api/files/lookup/ reports which MD5s are already in the library

Frontend:
- e621 client extended with post/tag/favorite types and helpers: post
  search, post detail, batch posts by id, tag autocomplete, toggle
  favorite
- /online: tag search with autocomplete, post grid with rating colors
  and in-library badges, numbered pagination, page tag cloud, blacklist
  panel and filtered counts from /users/me.json, anonymous hint
- /online/view/🆔 media viewer (sample or original, video support),
  taxonomy-colored tags by category, specs sheet, favorite/unfavorite,
  description, sources, pools and parent/children thumbnails
- Shared CollapsibleSidebar extracted from the library page; Online
  added to the nav, command palette and sidebar toggle

frontend/src/store/e621.ts

@@ -1,12 +1,19 @@
 import { create } from "zustand";
 
 import { api } from "@/lib/api";
-import type { E621Credentials } from "@/lib/e621";
+import {
+  effectiveCredentials,
+  verifyE621Credentials,
+  type E621Credentials,
+} from "@/lib/e621";
 
 interface E621State {
   credentials: E621Credentials | null;
+  blacklist: string[];
   loading: boolean;
+  ready: boolean;
   load: () => Promise<void>;
+  loadBlacklist: () => Promise<void>;
   save: (values: {
     username: string;
     api_key: string;
@@ -15,17 +22,35 @@ interface E621State {
   clear: () => void;
 }
 
-export const useE621 = create<E621State>((set) => ({
+export const useE621 = create<E621State>((set, get) => ({
   credentials: null,
+  blacklist: [],
   loading: false,
+  ready: false,
 
   load: async () => {
     set({ loading: true });
     try {
       const credentials = await api<E621Credentials>("/api/auth/e621/");
-      set({ credentials, loading: false });
+      set({ credentials, ready: true, loading: false });
+      void get().loadBlacklist();
     } catch {
-      set({ credentials: null, loading: false });
+      set({ credentials: null, ready: true, loading: false });
+    }
+  },
+
+  loadBlacklist: async () => {
+    try {
+      const user = await verifyE621Credentials(
+        effectiveCredentials(get().credentials),
+      );
+      const tags = (user.blacklisted_tags ?? "")
+        .split("\n")
+        .map((tag) => tag.trim())
+        .filter(Boolean);
+      set({ blacklist: tags });
+    } catch {
+      // Keep the last known blacklist when e621 is unreachable.
     }
   },
 
@@ -35,8 +60,9 @@ export const useE621 = create<E621State>((set) => ({
       json: values,
     });
     set({ credentials });
+    void get().loadBlacklist();
     return credentials;
   },
 
-  clear: () => set({ credentials: null }),
+  clear: () => set({ credentials: null, blacklist: [], ready: false }),
 }));