feat(home): add signup FAB for guest users

[isuri54]

Fixes #

Describe the changes you have made in this PR -
-Added a dedicated signup FloatingActionButton(FAB) for guest users
-Visible only when (isGuest == true)
-Routes to '/auth' on press for seamless onboarding
-Positioned at 'Alignment(0.7, -0.8)' to avoid UI clutter

Screenshots of the changes (If any) -
Due to a local environmental issue preventing the app from fully building, I couldn't capture actual screenshots. But here's what changed:
Before - Guest users had to navigate via logout dialog
After -
if (localApi.userModel.isGuest == true)
FloatingActionButton(
onPressed: () => appRouter.pushNamed('/auth'),
backgroundColor: kYellow,
child: Icon(Icons.login, color: Colors.white),
)

Note: Please let me know if screenshots are strictly needed; I can try fixing the build and update later.