BUG: Logged-in users are redirected to Login screen on every app launch

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

## 📌 Issue Overview
On every app launch, already authenticated users are redirected to the Login/Auth screen instead of being taken directly to their Home screen. The user must sign in again even though their session is still valid.

## 🔍 Steps to Reproduce
1. Install the Patient app and sign in with Google
2. Complete onboarding (personal details + assessment + consultation)
3. Close the app completely
4. Reopen the app
5. The Splash screen appears, then navigates to the Login/Auth screen - despite the user already being authenticated

## 🎯 Expected Behavior
If a valid Supabase session exists, the user should be routed directly to the Home screen (or whichever onboarding step is pending), skipping the Login screen entirely.

## 🚨 Actual Behavior
`SplashScreen._navigateToAuthScreen()` unconditionally navigates to `AuthScreen` after the 3-second delay, with no check for an existing session. This means every returning logged-in user is forced to sign in again on each launch.

**Root cause:** `splash_screen.dart` never calls `Supabase.instance.client.auth.currentSession` - it always pushes `AuthScreen` regardless of auth state.

## 📷 Screen Recording

https://github.com/user-attachments/assets/21b77570-f84e-448c-97ac-0908a5965ba1

## 💡 Suggested Improvements
In `SplashScreen`, after the splash delay, check `Supabase.instance.client.auth.currentSession`:
- If a session **exists** → route to the appropriate screen
- If **no session** → navigate to `AuthScreen` as before.

### Record

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- [x] I want to work on this issue