chore(hygiene): ignore editor artifacts; prettier SignerScreen

- .gitignore now covers .directory, .opencode/, .impeccable/.
- SignerScreen.tsx reformatted (format:check was failing since 81b082f).
- Dead untracked stub src/signer/nip46_external.rs deleted from disk
  (superseded by nip46_client.rs, never declared in signer/mod.rs).

.gitignore

@@ -6,6 +6,11 @@ profiles_vault.json
 profiles_vault.json.backup-*
 *.json.tmp
 
+# Editor / tool artifacts
+.directory
+.opencode/
+.impeccable/
+
 # Frontend
 frontend/node_modules/
 frontend/dist/