[Feature]: Add a "Bookmark" feature so users can save their favourite programs for quick access

## Summary

Many users of OpSo are actively researching which open-source programs to apply to. Currently, they must re-navigate to a program every time they want to reference it. A bookmark feature that persists across app sessions (using `shared_preferences`) would let users save and revisit their shortlisted programs without scrolling through the full list each time.

## Problem

- There is no way to mark or save a program as a favourite or bookmark.
- Users researching multiple programs have no way to quickly return to their shortlist.
- The app has no persistent local state — every session starts fresh.
- `shared_preferences` is already a common Flutter dependency and would not significantly increase bundle size.

## Proposed Solution

**Local persistence** using `shared_preferences` (add to `pubspec.yaml`):

```dart
// Save bookmark
final prefs = await SharedPreferences.getInstance();
List bookmarks = prefs.getStringList('bookmarks') ?? [];
if (!bookmarks.contains(programId)) {
bookmarks.add(programId);
await prefs.setStringList('bookmarks', bookmarks);
}
```

**UI changes:**

1. Add a bookmark icon (`Icons.bookmark_outline` / `Icons.bookmark`) to each program card on the home screen — toggles on tap.
2. Add a "Saved" tab or section at the top of the home screen that shows only bookmarked programs.
3. Show a filled bookmark icon on the program detail screen with a toggle.
4. Persist bookmark state across app restarts via `shared_preferences`.

**Files to modify:**

- `pubspec.yaml` — add `shared_preferences`
- Home screen widget — add bookmark icon to program card, add Saved tab
- Program detail screen — add bookmark toggle
- New file: `lib/services/bookmark_service.dart` — encapsulates all bookmark read/write logic

This is a self-contained feature with a clear scope. I'm happy to implement it end-to-end. Please assign this to me!

**Labels:** `enhancement`, `feature request`, `help wanted`, `GSSoC 2026`

Start by inspecting pubspec.yaml, the home screen program-card and Saved-section entry points, and the program detail screen. Add lib/services/bookmark_service.dart for persistent bookmark state, then verify that bookmark toggles appear on both screens, saved programs can be viewed, and state survives app restarts.

Written by the indexing model from the issue text.