fix: persist identity names per user instead of globally

Fixes issue where identity names were not saved when switching between
multiple identities. Names were being saved to master settings instead
of user-specific settings.

Changes:
- UserNameDialog: Load/save names from/to user-specific settings
- NewEditAccountView: Save names to user-specific settings for active DID
- Both components now use $accountSettings() and $saveUserSettings()
  instead of $settings() and $updateSettings()

Each identity now properly retains their assigned name when switching
between identities. Previously only "User Zero" would show their name
due to using master settings instead of per-identity settings.

Fixes: Identity name persistence across identity switches

+      // Save to user-specific settings for the current identity
+      await this.$saveUserSettings(activeDid, {
+        firstName: this.givenName,
+        lastName: "", // deprecated, pre v 0.1.3