feat(cli): configurable send-as From address (flags, TUI, validation)

- tui.ValidFromAddress: exported validator; blank passes, malformed rejects
- Fields.FromAddress: new field, round-trips through ToAccount/FieldsFromAccount
- Fields.Validate: calls ValidFromAddress before returning nil
- TUI form: from_address fieldDef between username and password
- send.go: From set via acc.SendFrom() instead of acc.Username
- admin.go account add: --from flag with pre-parse validation
- admin.go account edit: --from flag; validate before Visit, apply in Visit
- USER-MANUAL.md: --from flag added to account add flags table

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

internal/tui/account_test.go

@@ -157,3 +157,36 @@ func TestAccountFormCancel(t *testing.T) {
 		t.Fatal("esc should cancel the form")
 	}
 }
+
+func TestValidateRejectsBadFromAddress(t *testing.T) {
+	f := validFields()
+	f.FromAddress = "not an address"
+	if err := f.Validate(); err == nil {
+		t.Fatal("malformed from-address should fail validation")
+	}
+	f.FromAddress = "Steve Cliff <me@stevecliff.com>"
+	if err := f.Validate(); err != nil {
+		t.Fatalf("display-name from-address should validate: %v", err)
+	}
+	f.FromAddress = "me@stevecliff.com"
+	if err := f.Validate(); err != nil {
+		t.Fatalf("bare from-address should validate: %v", err)
+	}
+	f.FromAddress = "" // blank ⇒ fall back, always valid
+	if err := f.Validate(); err != nil {
+		t.Fatalf("blank from-address should validate: %v", err)
+	}
+}
+
+func TestFieldsFromToAccountCarriesFromAddress(t *testing.T) {
+	f := validFields()
+	f.FromAddress = "Steve Cliff <me@stevecliff.com>"
+	acc, _ := f.ToAccount()
+	if acc.FromAddress != "Steve Cliff <me@stevecliff.com>" {
+		t.Fatalf("ToAccount lost FromAddress: %q", acc.FromAddress)
+	}
+	back := FieldsFromAccount(acc)
+	if back.FromAddress != "Steve Cliff <me@stevecliff.com>" {
+		t.Fatalf("FieldsFromAccount lost FromAddress: %q", back.FromAddress)
+	}
+}