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/employees.js

@@ -54,7 +54,8 @@ export function renderEmployees() {
     if (!name) return;
     try {
       state.employees.push(createEmployee(name));
-    } catch {
+    } catch (err) {
+      console.warn('Mitarbeiter konnte nicht angelegt werden:', err);
       return;
     }
     nameInput.value = '';