fix: unify tender countdowns and add monthly calendar navigation

tests/calendar.test.mjs

@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { createTenderICS, tenderDeadline } from '../src/services/tenderCalendar.js';
+import { createTenderICS, tenderDeadline, deadlineInfo, calendarMonthDays, istanbulDateKey } from '../src/services/tenderCalendar.js';
 const t = {ikn:'2026/1582991',baslik:'Mikroskop',kurum:'Giresun',tarih:'10.09.2026',saat:'',aciklama:'Tarih ve Saati : 10.09.2026 - 10:00'};
 test('Giresun date is September 10 at 10:00 Turkey, not hard-coded August', () => {
   assert.equal(tenderDeadline(t).toISOString(),'2026-09-10T07:00:00.000Z');
@@ -23,3 +23,27 @@ test('ICS escapes text, folds UTF-8 lines, keeps stable UID', () => {
   assert.equal(ics.split('UID:')[1].split('\r\n')[0],'2026%2F1582991@ihaleist.com');
   assert.ok(ics.includes('\\nEND:VEVENT\\;\\,'));
 });
+
+test('all countdowns use Istanbul calendar days and exact expiry', () => {
+  const now = new Date('2026-09-06T08:00:00Z');
+  assert.equal(deadlineInfo(t,now).days,4);
+  assert.equal(deadlineInfo(t,now).label,'4 gün');
+  assert.equal(deadlineInfo(t,new Date('2026-09-10T06:59:00Z')).expired,false);
+  assert.equal(deadlineInfo(t,new Date('2026-09-10T07:00:00Z')).expired,true);
+  assert.equal(istanbulDateKey(new Date('2026-12-31T22:00:00Z')),'2027-01-01');
+});
+test('missing time stays unknown today, impossible date is not counted', () => {
+  const now=new Date('2026-09-10T10:00:00Z');
+  const info=deadlineInfo({tarih:'10.09.2026'},now);
+  assert.equal(info.expired,null);assert.equal(info.deadline,null);
+  assert.match(info.label,/saat doğrulanmalı/);
+  assert.equal(deadlineInfo({tarih:'31.02.2026'},now).days,null);
+  assert.equal(deadlineInfo({tarih:'09.09.2026'},now).expired,true);
+});
+test('month view includes late-month tenders and leap days', () => {
+  assert.equal(calendarMonthDays('2026-09').length,30);
+  assert.ok(calendarMonthDays('2026-09').some(d=>d.date==='22.09.2026'));
+  assert.equal(calendarMonthDays('2028-02').length,29);
+  assert.equal(calendarMonthDays('2027-02').length,28);
+  assert.deepEqual(calendarMonthDays('2026-13'),[]);
+});