fix(pdf): guard pdfSplit against non-positive interval infinite loop

The interval split mode looped for (i = 0; i < totalPages; i += interval),
which spins forever when interval <= 0. Both the single-file dialog and the
batch dialog can reach it (the batch dialog's validateOperationData only
checks truthiness, so -1 passes). Guard at the source in the main process:
reject non-positive or non-integer intervals before the loop, protecting
both paths and any future caller.

Amit Haridas

src/main/PDFOperations.js

@@ -112,6 +112,9 @@ async function pdfSplit(data) {
       }
     } else if (data.splitMode === 'interval') {
       const interval = data.interval;
+      if (!Number.isInteger(interval) || interval <= 0) {
+        return { success: false, message: 'Split interval must be a positive integer.' };
+      }
       for (let i = 0; i < totalPages; i += interval) {
         const pages = [];
         for (let j = i; j < i + interval && j < totalPages; j++) {