feat(music): add Kugou (酷狗音乐) as a music source (#69)

Adds a self-contained Kugou provider (bilibili-style: direct API calls, no
embedded API server, no new npm dependency) plus full backend + WebUI wiring.

Provider (src/music/kugou.ts): search, song-url (with device registration),
lyrics (KRC decode), song detail, playlist, album, personal FM, QR login +
cookie persistence, and quality. Request signing / crypto / KRC decoding are
ported from the MIT-licensed MakcRe/KuGouMusicApi using Node's built-in
crypto and zlib (no third-party crypto packages).

Wiring: the "kugou" platform is threaded through the provider contract, queue,
play-history, bot instance/manager dispatch (getProviderFor + the -k command
flag), index/server composition, the music/player/auth routers (unified
/search/all, /quality, the platform coercions, QR login), the cookie store,
and the WebUI (search source tab + badge, SongCard badge, brand token, and a
Kugou QR/cookie login card in Settings).

Verified live during development: search, lyrics, and album playback resolve
correctly. NOT verifiable in CI (Kugou anti-bot blocks the build host's IP):
play-URL resolution, QR login, and VIP audio — these are built faithfully to
the reference and need end-to-end testing on a non-flagged IP / a Kugou
account. See the header comment in kugou.ts.

Includes src/music/kugou.test.ts (mappers + KRC→LRC). An adversarial review
pass fixed: pagination truncating on filtered counts, an ms/seconds duration
heuristic, dfid soft-fail caching, the /v5/url random-dfid fallback, the FM
body identity, and an unguarded nickname decode.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/music/auth.ts

@@ -2,8 +2,8 @@ import fs from "node:fs";
 import path from "node:path";
 
 export interface CookieStore {
-  save(platform: "netease" | "qq" | "bilibili", cookie: string): void;
-  load(platform: "netease" | "qq" | "bilibili"): string;
+  save(platform: "netease" | "qq" | "bilibili" | "kugou", cookie: string): void;
+  load(platform: "netease" | "qq" | "bilibili" | "kugou"): string;
 }
 
 export function createCookieStore(cookieDir: string): CookieStore {
@@ -12,7 +12,7 @@ export function createCookieStore(cookieDir: string): CookieStore {
   }
 
   return {
-    save(platform: "netease" | "qq" | "bilibili", cookie: string): void {
+    save(platform: "netease" | "qq" | "bilibili" | "kugou", cookie: string): void {
       const filePath = path.join(cookieDir, `${platform}.json`);
       fs.writeFileSync(
         filePath,
@@ -21,7 +21,7 @@ export function createCookieStore(cookieDir: string): CookieStore {
       );
     },
 
-    load(platform: "netease" | "qq" | "bilibili"): string {
+    load(platform: "netease" | "qq" | "bilibili" | "kugou"): string {
       const filePath = path.join(cookieDir, `${platform}.json`);
       if (!fs.existsSync(filePath)) return "";
       try {