feat: add a "Deadline Countdown" and calendar reminder feature to program detail screens — users currently see application dates as static text but have no way to track upcoming deadlines or add them to their device calendar

## 🚀 Problem Statement

Open-source programs like GSoC, LFX, and Outreachy have **strict application deadlines** that students frequently miss. OpSo shows dates as static text on program detail screens, but:

1. There is no visual countdown showing "Application closes in 12 days"
2. There is no way to add a deadline reminder to the device calendar
3. There is no in-app notification system for upcoming deadlines
4. Missed deadlines = missed opportunities — this is exactly the problem OpSo should solve

For an app whose purpose is helping developers discover and apply to programs, deadline awareness is the single most impactful missing feature.

## ✅ Proposed Solution

### 1. Create a `DeadlineCountdown` widget

````dart
// lib/components/deadline_countdown.dart
import 'package:flutter/material.dart';
import 'dart:async';

class DeadlineCountdown extends StatefulWidget {
final DateTime deadline;
final String label;

const DeadlineCountdown({Key? key, required this.deadline, required this.label})
: super(key: key);

@override
State createState() => _DeadlineCountdownState();
}

class _DeadlineCountdownState extends State {
late Timer _timer;
late Duration _remaining;

@override
void initState() {
super.initState();
_remaining = widget.deadline.difference(DateTime.now());
_timer = Timer.periodic(const Duration(seconds: 1), (_) {
setState(() {
_remaining = widget.deadline.difference(DateTime.now());
});
});
}

@override
void dispose() { _timer.cancel(); super.dispose(); }

Color get urgencyColor {
if (_remaining.inDays < 3) return Colors.red;
if (_remaining.inDays < 7) return Colors.orange;
return Colors.green;
}

@override
Widget build(BuildContext context) {
if (_remaining.isNegative) {
return Chip(
label: const Text('Deadline Passed'),
backgroundColor: Colors.grey[300],
);
}

return Container(
padding: const EdgeInsets.all(12),
decoration: BoxDecoration(
color: urgencyColor.withOpacity(0.1),
border: Border.all(color: urgencyColor),
borderRadius: BorderRadius.circular(8),
),
child: Row(children: [
Icon(Icons.timer, color: urgencyColor),
const SizedBox(width: 8),
Text(
'${_remaining.inDays}d ${_remaining.inHours % 24}h ${_remaining.inMinutes % 60}m',
style: TextStyle(color: urgencyColor, fontWeight: FontWeight.bold, fontSize: 18),
),
const Spacer(),
Text(widget.label, style: TextStyle(color: urgencyColor)),
]),
);
}
}
````

### 2. Add "Add to Calendar" button using `add_2_calendar` package

````yaml
# pubspec.yaml
dependencies:
add_2_calendar: ^3.0.1
````

````dart
// lib/components/add_to_calendar_button.dart
import 'package:add_2_calendar/add_2_calendar.dart';

void addDeadlineToCalendar({
required String programName,
required DateTime deadline,
required String description,
}) {
final Event event = Event(
title: '$programName — Application Deadline',
description: description,
location: 'Online',
startDate: deadline.subtract(const Duration(hours: 1)),
endDate: deadline,
allDay: false,
);
Add2Calendar.addEvent2Cal(event);
}
````

### 3. Integrate into program detail screens

````dart
// In each program's detail screen
Column(children: [
DeadlineCountdown(
deadline: program.applicationDeadline,
label: 'Applications Close',
),
const SizedBox(height: 8),
ElevatedButton.icon(
icon: const Icon(Icons.calendar_today),
label: const Text('Add Reminder to Calendar'),
onPressed: () => addDeadlineToCalendar(
programName: program.name,
deadline: program.applicationDeadline,
description: 'Apply at: ${program.website}',
),
),
])
````

## 📁 Files to Modify

| File | Change |
|---|---|
| `pubspec.yaml` | Add `add_2_calendar` dependency |
| `lib/components/deadline_countdown.dart` | Create animated countdown widget |
| `lib/components/add_to_calendar_button.dart` | Create calendar integration button |
| All program detail screens in `lib/screens/` | Integrate deadline + calendar button |

**Suggested labels:** `enhancement`, `flutter`, `ux`, `feature`, `level: beginner`

I would like to work on this. Could you please assign it to me?