Add OTP login flow and multi-cafe role switching

Introduce an OTP input box on login/register, surface user roles and a
cafe chooser, add a dashboard switch button in the POS screen, and
register OTP validators explicitly to survive Docker layer caching.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Meezi.API/Models/Auth/AuthDtos.cs

@@ -6,12 +6,17 @@ public record VerifyOtpRequest(string Phone, string Code, string? CafeId = null)
 
 public record RefreshTokenRequest(string RefreshToken);
 
+public record SwitchCafeRequest(string CafeId);
+
 /// <summary>Step 1 of self-registration: send OTP to a new phone number.</summary>
 public record RegisterRequest(string Phone, string CafeName);
 
 /// <summary>Step 2 of self-registration: verify OTP and create the cafe account.</summary>
 public record VerifyRegisterRequest(string Phone, string Code);
 
+/// <summary>One café membership entry returned when user belongs to multiple cafés.</summary>
+public record CafeMembershipDto(string CafeId, string CafeName, string Role, string PlanTier);
+
 public record AuthTokenResponse(
     string AccessToken,
     string RefreshToken,
@@ -22,6 +27,10 @@ public record AuthTokenResponse(
     string PlanTier,
     string Language,
     string Actor = Meezi.Core.Constants.MeeziActorKinds.Merchant,
-    string? BranchId = null);
+    string? BranchId = null,
+    List<CafeMembershipDto>? Memberships = null);
 
 public record SendOtpResponse(bool Sent, int ExpiresInSeconds);
+
+/// <summary>Returned when a phone number belongs to multiple cafés and no CafeId was specified.</summary>
+public record CafeChoicesResponse(List<CafeMembershipDto> Cafes);