Remove legacy auth cache migration docs and add regression test

Removes obsolete "Migration notes" from `docs/nostr-auth.md` and
adds a regression test to `tests/state/cache-saved-profiles.test.mjs`
to confirm that the legacy `userPubKey` localStorage key is ignored
during startup. Also polyfills localStorage for the test suite.

docs/nostr-auth.md

@@ -59,16 +59,6 @@ When introducing additional providers, choose a unique string value and extend t
 When the app reads stored entries it normalises unknown values back to
 `"nip07"` but keeps recognised alternatives intact.
 
-## Migration notes
+## Supported Auth Cache
 
-Earlier builds only persisted a single `userPubKey` string. During startup the
-app now:
-
-1. Attempts to read `bitvid:savedProfiles:v1` and validate the payload.
-2. If the key is missing (or malformed) but a legacy `userPubKey` entry exists,
-   it seeds `savedProfiles` with that value and writes the new structure.
-3. Once the JSON payload is written successfully, the legacy `userPubKey` entry
-   is removed.
-
-Future migrations should follow the same pattern: validate, normalise, write the
-new format, then clean up legacy keys to avoid data loss.
+`bitvid:savedProfiles:v1` is the sole supported auth cache. Legacy keys such as `userPubKey` are no longer read or migrated.