Let rules carry a note

A glob like *4412* says nothing about why it exists or who it catches, and
six months later neither does memory. Rules get an optional note: free text
that never takes part in matching, written as a TOML key rather than a
comment so it survives a round trip and can be shown back.

The rule builder grows a fourth field for it and the rules screen a last
column. Four fields spaced out are taller than a short window has room for,
so the form now drops its blank lines and then the hints on unfocused fields
before anything would run off the bottom.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/config/config.go

@@ -38,6 +38,11 @@ type Rule struct {
 	// Type matches the bank's own classification, e.g. Revolut's CARD_PAYMENT.
 	// Optional; when set, it must match as well as Match.
 	Type string `toml:"type"`
+	// Note is free text for the reader: why the rule exists, or what the
+	// unrecognisable payee behind the glob actually is. It never affects
+	// matching. It is a key rather than a comment so it survives a round trip
+	// through LoadRules and can be shown on the rules screen.
+	Note string `toml:"note"`
 }
 
 // Rules is the parsed rules.toml.
@@ -256,6 +261,9 @@ func formatRule(r Rule) string {
 	if r.Transfer {
 		b.WriteString("transfer     = true\n")
 	}
+	// Last, so the patterns and the tag stay lined up above it however long
+	// the note runs.
+	write("note", r.Note)
 	return b.String()
 }