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/cli/send_test.go

@@ -139,6 +139,32 @@ func TestSendReplyToThreadsHeaders(t *testing.T) {
 	}
 }
 
+func TestSendUsesConfiguredFromAddress(t *testing.T) {
+	acc := rwAccount()
+	acc.FromAddress = "Steve Cliff <me@stevecliff.com>"
+	d, sent, _ := sendDeps(t, acc, nil)
+	if err := SendCmd(d, "send", []string{"me@stevecliff.com"}, nil, nil, "hi", "body", nil, 0, "INBOX"); err != nil {
+		t.Fatalf("SendCmd: %v", err)
+	}
+	if len(*sent) != 1 {
+		t.Fatalf("want 1 send, got %d", len(*sent))
+	}
+	if got := (*sent)[0].From; got != "Steve Cliff <me@stevecliff.com>" {
+		t.Fatalf("From = %q, want configured from-address", got)
+	}
+}
+
+func TestSendFallsBackToUsernameAsFrom(t *testing.T) {
+	// rwAccount has no FromAddress, so From must be the login username.
+	d, sent, _ := sendDeps(t, rwAccount(), nil)
+	if err := SendCmd(d, "send", []string{"me@stevecliff.com"}, nil, nil, "hi", "body", nil, 0, "INBOX"); err != nil {
+		t.Fatalf("SendCmd: %v", err)
+	}
+	if got := (*sent)[0].From; got != "emcli@stevecliff.com" {
+		t.Fatalf("From = %q, want username fallback", got)
+	}
+}
+
 func TestSendReplyToFilteredSourceNotFound(t *testing.T) {
 	acc := rwAccount()
 	acc.WhitelistInEnabled = true // inbound filter active