feat(store): add account from_address field + v2 migration

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/store/account_test.go

@@ -84,3 +84,30 @@ func TestListAccountsOmitsSecrets(t *testing.T) {
 		t.Fatal("ListAccounts must not return secrets")
 	}
 }
+
+func TestSendFromFallsBackToUsername(t *testing.T) {
+	a := Account{Username: "login@example.com"}
+	if got := a.SendFrom(); got != "login@example.com" {
+		t.Fatalf("blank from-address should fall back to username, got %q", got)
+	}
+	a.FromAddress = "Steve Cliff <me@stevecliff.com>"
+	if got := a.SendFrom(); got != "Steve Cliff <me@stevecliff.com>" {
+		t.Fatalf("set from-address should win, got %q", got)
+	}
+}
+
+func TestAddGetAccountRoundTripsFromAddress(t *testing.T) {
+	s := openTemp(t)
+	a := sampleAccount()
+	a.FromAddress = "Steve Cliff <me@stevecliff.com>"
+	if _, err := s.AddAccount(a); err != nil {
+		t.Fatalf("AddAccount: %v", err)
+	}
+	got, err := s.GetAccount("work")
+	if err != nil {
+		t.Fatalf("GetAccount: %v", err)
+	}
+	if got.FromAddress != "Steve Cliff <me@stevecliff.com>" {
+		t.Fatalf("FromAddress not round-tripped: %q", got.FromAddress)
+	}
+}