PAINTROID-723: Fix CircularProgressIndicator animation on landing page

[aryanorastar]

Ticket: https://catrobat.atlassian.net/browse/PAINTROID-723

New Features and Enhancements

• None

Refactorings and Bug Fixes

• Landing page passed a new _getProjects() future into FutureBuilder on every rebuild, which restarted CircularProgressIndicator and made the spinner look stuck / non-animated.
• Cache the projects Future on State; recreate it when returning from Pocket Paint and when ProjectDatabase is replaced after invalidate (delete/rename).
• Widget test: while the list future is pending, rebuilds do not call getProjects() again.

Checklist

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Verification

• flutter test test/widget/landing_page/landing_page_test.dart — 21/21 passed
• Physical device / emulator animation smoke not run here