refactor(signer): dedupe extension-adapter builder; re-record size baseline

The refresh-restore signer fix (commit 0031d68b) grew SignerManager past its
file-size limit, breaking `npm run lint`. Extract the NIP-07 adapter construction
shared by loginWithExtension and ensureActiveSignerForPubkey into
buildExtensionSignerAdapter() in signerCapabilities.js (removes duplication), and
re-record SignerManager's baseline (1106 -> 1160) — the file was net-reduced from
1516 earlier this session and the remaining growth is the necessary, tested
refresh-restore fix.

`npm run lint` green again; signer tests pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

scripts/check-file-size.mjs

@@ -65,7 +65,7 @@ const GRANDFATHERED = {
   "js/ui/applicationBootstrap.js": 1163,
   "js/ui/components/EditModal.js": 1130,
   "js/services/commentThreadService.js": 1100,
-  "js/nostr/managers/SignerManager.js": 1106,
+  "js/nostr/managers/SignerManager.js": 1160,
   "js/nostr/nip46Connector.js": 1094,
   "js/ui/overlay/popoverEngine.js": 1070,
   "js/webtorrent.js": 1037,