feat(splits): save split as rule from split modal

- Checkbox in split modal: 'Also save as rule for <merchant>'
- Creates a rule with apply_split action storing the participant shares
- Rules engine now handles apply_split: deletes existing splits and re-applies
- Bulk split mode hides the checkbox (rule wouldn't make sense for ad-hoc bulk)

src/lib/hooks.ts

@@ -360,7 +360,7 @@ export interface RuleRow {
   id: number;
   name: string;
   conditions: { field: string; operator: string; value: string }[];
-  actions: { set_category?: string; add_tag_ids?: number[]; set_merchant?: string };
+  actions: { set_category?: string; add_tag_ids?: number[]; set_merchant?: string; apply_split?: { participant_id: number; share_percent: number }[] };
   enabled: boolean;
   priority: number;
   created_at: string;