fix(settings): read the v0.3 username response shape

The v0.3 backend answers GET /users/me/username with {"username": ...};
the legacy backend sent a bare string. The page put the object into state
and rendered it, so /settings threw for every signed-in user.

parseUsernameResponse accepts both shapes and returns only a string or
null. Legacy-only fields on /auth/me are optional. Adds node:test cases,
run with npm test.

tsconfig.json

@@ -10,6 +10,7 @@
     "skipLibCheck": true,
     "strict": true,
     "noEmit": true,
+    "allowImportingTsExtensions": true,
     "esModuleInterop": true,
     "module": "esnext",
     "moduleResolution": "bundler",