fix(robustness): parseInt radix, minGapDays null-guard, log silent catches (#14)

- parseInt(month/year) now passes radix 10 consistently.
- minGap eligibility guards null/undefined (c.minGapDays != null && > 0).
- The corrupt-localStorage load and the createEmployee catch now console.warn
  instead of swallowing the error silently.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/calendar.js

@@ -99,13 +99,13 @@ export function renderCalendar() {
   renderGrid(wrapper, exportBtn);
 
   monthSel.addEventListener('change', () => {
-    currentMonth = parseInt(monthSel.value);
+    currentMonth = parseInt(monthSel.value, 10);
     loadPlanFromHistory();
     renderGrid(wrapper, exportBtn);
   });
 
   yearSel.addEventListener('change', () => {
-    currentYear = parseInt(yearSel.value);
+    currentYear = parseInt(yearSel.value, 10);
     loadPlanFromHistory();
     renderGrid(wrapper, exportBtn);
   });