refactor(profile): make UserProfile.displayName non-nullable

[bmc08gt]

What

UserProfile.displayName was declared nullable but the backing proto always provides a value (empty string when unset, never absent). This makes the field non-nullable to match, then cleans up the fallout.

Changes

• Model: UserProfile.displayName: String? → String; Empty.displayName = "".
• Construction sites updated to satisfy the non-null contract:
  • ProfileController — fallback now uses UserProfile.Empty.copy(...).
  • ChatEntityMapper — member fallback uses UserProfile.Empty; serialized→domain coerces cached nullable name via orEmpty().
  • ProfileCoordinator — cached (nullable) name coerced via orEmpty() when mapping to domain.
• Redundant null handling removed where the receiver is a non-null UserProfile:
  • ChatParticipant — profile.displayName.orEmpty() → profile.displayName.
  • ChatViewModel / FeedSyncDelegate — !displayName.isNullOrBlank() → displayName.isNotBlank().
  • ContactListBuilder, UserProfileScreenContent, UserProfileViewModel, TipFlowViewModel — dropped ?/elvis at non-null sites.
• Tests updated for the non-null field; removed a now-impossible "null display name" screen test that duplicated the existing "empty" case.

Sites where displayName is reached through a nullable profile/member were intentionally left as-is — their nullability comes from the receiver, not the field.

Testing

• compileDebugKotlin green across all affected modules.
• myaccount debug unit-test sources compile.