Count hand-valued assets into the wealth figure

A wealth figure that ignores the house is not a wealth figure. Assets
without a market feed - a house, a car, a private loan - are now added
by hand on the Wealth page with a stated value, a currency and the day
the estimate was made; a negative value records a liability. They are
registry entities in assets.finance like everything else, join the
per-currency totals immediately, and a currency held only in an asset
earns its own line.

internal/app/manage.go

@@ -104,6 +104,25 @@ func SaveInstrument(d *domain.Dataset, v domain.Instrument) error {
 	d.Instruments = append(d.Instruments, v)
 	return nil
 }
+
+// SaveAsset registers or revalues a hand-valued possession. The value and the
+// day it was stated travel together; full validation happens at commit.
+func SaveAsset(d *domain.Dataset, v domain.Asset) error {
+	v.Name = strings.TrimSpace(v.Name)
+	v.Kind = strings.TrimSpace(v.Kind)
+	v.Currency = strings.ToUpper(strings.TrimSpace(v.Currency))
+	if v.ID == "" {
+		v.ID = domain.NewID("asset")
+	}
+	for i, x := range d.Assets {
+		if x.ID == v.ID {
+			d.Assets[i] = v
+			return nil
+		}
+	}
+	d.Assets = append(d.Assets, v)
+	return nil
+}
 func SaveCategory(d *domain.Dataset, v domain.Category) error {
 	v.Name = strings.TrimSpace(v.Name)
 	if v.ID == "" {
@@ -203,6 +222,15 @@ func Manage(d *domain.Dataset, entity, action, id, target string) error {
 		if n == len(d.Instruments) {
 			return errors.New("unknown instrument")
 		}
+	case "asset":
+		if action != "delete" {
+			return errors.New("asset merging is not supported")
+		}
+		n := len(d.Assets)
+		d.Assets = slices.DeleteFunc(d.Assets, func(v domain.Asset) bool { return v.ID == id })
+		if n == len(d.Assets) {
+			return errors.New("unknown asset")
+		}
 	case "tag":
 		if !slices.ContainsFunc(d.Tags, func(v domain.Tag) bool { return v.ID == id }) {
 			return errors.New("unknown tag")