Fix F-07: pre-compute real Argon2 dummy hash via sync.Once

- auth/auth.go: add DummyHash() which uses sync.Once to compute
  HashPassword("dummy-password-for-timing-only", DefaultArgonParams())
  on first call; subsequent calls return the cached PHC string;
  add sync to imports
- auth/auth_test.go: TestDummyHashIsValidPHC verifies the hash
  parses and verifies correctly; TestDummyHashIsCached verifies
  sync.Once behaviour; TestDummyHashMatchesDefaultParams verifies
  embedded m/t/p match DefaultArgonParams()
- server/server.go, grpcserver/auth.go, ui/ui.go: replace five
  hardcoded PHC strings with auth.DummyHash() calls
- AUDIT.md: mark F-07 as fixed
Security: the previous hardcoded hash used a 6-byte salt and
  6-byte output ("testsalt"/"testhash" in base64), which Argon2id
  verifies faster than a real 16-byte-salt / 32-byte-output hash.
  This timing gap was measurable and could aid user enumeration.
  auth.DummyHash() uses identical parameters and full-length salt
  and output, so dummy verification timing matches real timing
  exactly, regardless of future parameter changes.

internal/grpcserver/auth.go

@@ -40,14 +40,14 @@ func (a *authServiceServer) Login(ctx context.Context, req *mciasv1.LoginRequest
 	acct, err := a.s.db.GetAccountByUsername(req.Username)
 	if err != nil {
 		// Security: run dummy Argon2 to equalise timing for unknown users.
-		_, _ = auth.VerifyPassword("dummy", "$argon2id$v=19$m=65536,t=3,p=4$dGVzdHNhbHQ$dGVzdGhhc2g")
+		_, _ = auth.VerifyPassword("dummy", auth.DummyHash())
 		a.s.db.WriteAuditEvent(model.EventLoginFail, nil, nil, ip, //nolint:errcheck // audit failure is non-fatal
 			fmt.Sprintf(`{"username":%q,"reason":"unknown_user"}`, req.Username))
 		return nil, status.Error(codes.Unauthenticated, "invalid credentials")
 	}
 
 	if acct.Status != model.AccountStatusActive {
-		_, _ = auth.VerifyPassword("dummy", "$argon2id$v=19$m=65536,t=3,p=4$dGVzdHNhbHQ$dGVzdGhhc2g")
+		_, _ = auth.VerifyPassword("dummy", auth.DummyHash())
 		a.s.db.WriteAuditEvent(model.EventLoginFail, &acct.ID, nil, ip, `{"reason":"account_inactive"}`) //nolint:errcheck
 		return nil, status.Error(codes.Unauthenticated, "invalid credentials")
 	}