feat: support unassigned purchases and allocation status

Allow month-only purchases to start finance plans. Keep unbudgeted posts distinct from explicit zero rules in overspend reporting.

src/ui/components/RuleHistory.tsx

@@ -53,8 +53,8 @@ export function RuleHistory({ post }: Props) {
     <div className="space-y-3 rounded-md border border-budget-rule p-3">
       {post.rules.length === 0 ? (
         <p className="text-xs text-muted-foreground">
-          Not budgeted. Until a rule starts, this post is allocated nothing —
-          spending still records, and shows as overspend.
+          Not budgeted. Until a rule starts, this post is allocated nothing.
+          Spending still records without an overspend status.
         </p>
       ) : (
         <table className="w-full text-xs">