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/SplitEditor.tsx

@@ -39,6 +39,26 @@ export function SplitEditor({ draft, posts, onChange }: Props) {
 
   return (
     <fieldset className="space-y-3">
+      <label className="flex items-center gap-2 text-sm">
+        <input
+          type="checkbox"
+          checked={draft.splits.length === 0}
+          onChange={(event) =>
+            onChange({
+              ...draft,
+              splits: event.target.checked
+                ? []
+                : posts[0]
+                  ? [{ postId: posts[0].id, value: 100, absorbsRemainder: true }]
+                  : [],
+            })
+          }
+          disabled={posts.length === 0 && draft.splits.length === 0}
+        />
+        No post — leave this purchase unassigned
+      </label>
+      {draft.splits.length > 0 && (
+        <>
       <div className="flex items-center justify-between">
         <Label>Split across posts</Label>
         <Segmented
@@ -136,6 +156,8 @@ export function SplitEditor({ draft, posts, onChange }: Props) {
             : `${fmt.amount(balance)} ${unit} unassigned — the rounding post absorbs it.`}
         </span>
       </div>
+        </>
+      )}
     </fieldset>
   );
 }