feat: add offline support using `hive` local database — OpSo currently requires internet connectivity to display program information, showing a blank screen with no cached data when offline

## 🚀 Problem Statement

OpSo is a **discovery app** — its primary function is providing information about open-source programs. This information (program descriptions, dates, eligibility, etc.) is largely **static content that changes infrequently**. Yet:

- If a student opens OpSo on an airplane or in a low-connectivity area, they see a loading spinner or blank screen
- Program details they were just reading are not available offline
- There is no local caching mechanism anywhere in the codebase
- The app does not function as a reliable reference tool

For a tool meant to help developers prepare for program applications, being unavailable offline is a serious UX gap.

## ✅ Proposed Solution

Use `hive` (lightweight, fast Flutter key-value database) to cache program data locally with a 24-hour TTL.

### 1. Add dependencies to `pubspec.yaml`

````yaml
dependencies:
hive: ^2.2.3
hive_flutter: ^1.1.0
connectivity_plus: ^6.0.3

dev_dependencies:
hive_generator: ^2.0.1
build_runner: ^2.4.9
````

### 2. Create cache service `lib/services/cache_service.dart`

````dart
// lib/services/cache_service.dart
import 'package:hive_flutter/hive_flutter.dart';

class CacheService {
static const _programBox = 'programs_cache';
static const _metaBox = 'cache_meta';
static const cacheTTL = Duration(hours: 24);

static Future init() async {
await Hive.initFlutter();
await Hive.openBox(_programBox);
await Hive.openBox(_metaBox);
}

static Future cachePrograms(String key, List> data) async {
final box = Hive.box(_programBox);
await box.put(key, data);
await Hive.box(_metaBox).put('${key}_timestamp', DateTime.now().toIso8601String());
}

static List>? getCachedPrograms(String key) {
final metaBox = Hive.box(_metaBox);
final timestampStr = metaBox.get('${key}_timestamp') as String?;
if (timestampStr == null) return null;

final timestamp = DateTime.parse(timestampStr);
if (DateTime.now().difference(timestamp) > cacheTTL) return null; // expired

final box = Hive.box(_programBox);
final data = box.get(key);
return data != null ? List>.from(data) : null;
}

static bool isCacheValid(String key) {
return getCachedPrograms(key) != null;
}
}
````

### 3. Create connectivity-aware data provider

````dart
// lib/providers/program_provider.dart
import 'package:connectivity_plus/connectivity_plus.dart';

class ProgramProvider extends ChangeNotifier {
List _programs = [];
bool _isOffline = false;
bool _isLoading = false;

bool get isOffline => _isOffline;

Future loadPrograms() async {
_isLoading = true;
notifyListeners();

final connectivity = await Connectivity().checkConnectivity();
_isOffline = connectivity == ConnectivityResult.none;

if (_isOffline) {
// Load from cache
final cached = CacheService.getCachedPrograms('all_programs');
if (cached != null) {
_programs = cached.map(ProgramModel.fromMap).toList();
}
} else {
// Fetch fresh data and update cache
final freshData = await ProgramRepository.fetchAll();
_programs = freshData;
await CacheService.cachePrograms('all_programs', freshData.map((p) => p.toMap()).toList());
}

_isLoading = false;
notifyListeners();
}
}
````

### 4. Show offline banner in the home screen

````dart
// lib/screens/home_screen.dart
if (provider.isOffline)
Container(
width: double.infinity,
color: Colors.orange[100],
padding: const EdgeInsets.symmetric(vertical: 6, horizontal: 16),
child: Row(children: [
const Icon(Icons.wifi_off, size: 16, color: Colors.orange),
const SizedBox(width: 8),
const Text('You\'re offline — showing cached data', style: TextStyle(color: Colors.orange)),
]),
),
````

## 📁 Files to Create / Modify

| File | Change |
|---|---|
| `pubspec.yaml` | Add `hive`, `hive_flutter`, `connectivity_plus` |
| `lib/services/cache_service.dart` | Create Hive cache service |
| `lib/providers/program_provider.dart` | Add offline/online data loading |
| `lib/main.dart` | Initialize Hive on startup |
| `lib/screens/home_screen.dart` | Show offline banner when cache used |

**Suggested labels:** `enhancement`, `flutter`, `offline`, `performance`, `level: intermediate`

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