BUG: Daily activity completions are not saved to Supabase, progress resets on navigation

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

## 📌 Issue Overview

When a patient checks off daily activities as completed, the checkbox toggles only update local in-memory state and are never written to Supabase. All progress is silently lost the moment the user navigates away, switches dates, or restarts the app.

Three interacting defects cause this:

1. `toggleTaskCompletion()` in `task_provider.dart` only mutates `_allTasks` in memory and calls `notifyListeners()`. It never calls `updateActivityCompletion()`. The method is marked `async` but contains no `await` and is effectively synchronous.

2. The only fallback save path was explicitly commented out in `daily_activities_screen.dart`:
```dart
// context.read().updateActivityInBackground(); // TODO: Uncomment this when the backend is ready
```
The method `updateActivityInBackground` does not even exist on `TaskProvider`, it was never implemented.

3. `updateActivityCompletion()` is only called inside `setSelectedDate()`, meaning the only time data is saved is when the user switches the date picker to a different day. If the user checks activities and presses back without changing dates, nothing is saved. This also introduces a race condition since `getTodayActivities()` is called immediately
after, potentially overwriting unsaved state with a fresh DB read.

The screen has no `PopScope`, no `WillPopScope`, no `AppLifecycleObserver`, and no
`dispose()` save logic. There is zero mechanism to flush pending changes on any form of exit.

## 🔍 Steps to Reproduce
1. Log in as a patient who has daily activities assigned
2. Navigate to **Home → Daily Activities**
3. Wait for activities to load
4. Check 2 or more activity checkboxes - observe they toggle visually
5. Press the back arrow to return to home
6. Navigate back to **Daily Activities**
7. All checkboxes are unchecked, all progress is lost
8. Alternatively: check activities, kill and restart the app, same result

## 🎯 Expected Behavior

Each checkbox toggle should persist the `is_completed` state to the `daily_activity_logs` table in Supabase immediately or on a short debounce. Navigating away and back should restore the previously checked state from the database.

## 🚨 Actual Behavior

No Supabase call is made on checkbox toggle. The in-memory state is discarded on navigation. The `daily_activity_logs` table never reflects actual patient completions, which means:
- The Reports screen always shows activities as incomplete
- Therapists see 0% completion for activities the patient believes they completed
- Patient therapy progress data is silently corrupted

## 📷 Screenshot

Not applicable. The bug is in data not written to Supabase, not visible in the UI.

## 💡 Suggested Improvements

**`task_provider.dart`:**
- Add a `Timer? _debounceTimer` field
- In `toggleTaskCompletion()`, after mutating `_allTasks`, start a 1.5s debounce timer that calls `updateActivityCompletion(_allTasks)`
- Add a `Future saveAndFlush()` method that cancels the timer and immediately
awaits `updateActivityCompletion(_allTasks)` for use on screen exit

**`daily_activities_screen.dart`:**
- Wrap the `Scaffold` in a `PopScope` with `onPopInvokedWithResult` calling `taskProvider.saveAndFlush()` before allowing pop
- Add `WidgetsBindingObserver` and implement `didChangeAppLifecycleState` to call `saveAndFlush()` on `AppLifecycleState.paused`
- Remove the commented-out `updateActivityInBackground` line

**`supabase_patient_repository.dart`:**
- No changes needed. `updateActivityCompletion()` is correctly implemented, it is just never called from the right place

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this issue