Move QgisMobileapp into src/app behind an AppController interface

[MidnightCoder-m]

🚀 Description

Moves QgisMobileapp out of src/core into a new src/app library (qfield_app), removing the last core → app dependency.

The only obstacle was AppInterface (which stays in core) reaching intoQgisMobileapp via qobject_cast on the QML engine. That's inverted with asmall abstract interface AppController (in core): QgisMobileapp implements it and is now handed to AppInterface explicitly, so core no longer knows the concrete app class.

What's next

• gui layer — add src/gui (qfield_gui), move editing/attribute-form UX classes out of core
• QML modules — adopt QML_ELEMENT
• plugin — bundle core+gui as the QField plugin; app becomes a pure consumer

[qfield-fairy]

🍎 MacOS DMG universal builds

Download a MacOS DMG universal build of this PR for testing.
(Built from commit 24c27ba)

📱 Android builds

Download an Android arm64 build of this PR for testing.
(Built from commit 24c27ba)

Other Android architectures

• arm-neon
• x64
• x86
• all access arm64
• all access arm-neon
• all access x64
• all access x86

🐧 Linux AppImage builds

Download a Linux AppImage build of this PR for testing.
(Built from commit 24c27ba)

🪟 Windows builds

Download a Windows build of this PR for testing and for arm64.

(Built from commit 24c27ba)

[nirvn]

Sweet!