Fixes MV3 service worker startup failures caused by stale/duplicate context menu IDs. Context menus now rebuild with a full removeAll pass, overlapping tab group rebuilds are serialized, and duplicate create errors are ignored during rebuild.\n\nValidation:\n- npm test -- test/background.test.ts: 81 passed\n- npm test: 394 passed\n- npm run build: passed\n- npm run package: produced gtabs-extension.zip\n\nGitHub asset SHA-256: a6816dbed6bd6b47f265d0463896d46f23721b106e4637669e548bca5d89453e