Greatest: serve catalog covers from a local copy (BL-0053)

The page hotlinked ~100 AniList CDN covers per load and some failed at random.
The reader now downloads each catalog cover once (sequential, ~1/s, stops on
401/403/429, images up to 2 MB) into data/greatest-covers/ and serves it from
/api/greatest-cover with a year-long browser cache; until a copy exists the
route redirects to the catalog URL. The page also accepts the server's
relative cover URLs, so owned titles show their library cover again.
GREATEST_COVER_WARM=0 turns the downloads off (tests, offline installs).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/test-greatest-ui.js

@@ -40,7 +40,7 @@ async function freePort() {
   const scraperPort = fakeScraper.address().port;
   const child = spawn(process.execPath, ['server.js'], { cwd: root, windowsHide: true,
     env: { ...process.env, PORT: String(port), LIBRARY: library, DATA: data, SCAN_INTERVAL: '0',
-      FULL_SCAN_INTERVAL: '0', SCRAPER_URL: `http://127.0.0.1:${scraperPort}`, COOKIE_SECURE: '0' }, stdio: 'ignore' });
+      FULL_SCAN_INTERVAL: '0', SCRAPER_URL: `http://127.0.0.1:${scraperPort}`, COOKIE_SECURE: '0', GREATEST_COVER_WARM: '0' }, stdio: 'ignore' });
   let browser;
   try {
     for (let i = 0; i < 80; i++) {