T1.8: Delete empty src/core; move version to src/app

Move version.go from src/core to src/app to prepare for deleting the
now-empty src/core package. Update all imports and build script
references to use src/app.Version instead of src/core.Version.

Fix import aliasing conflict between Fyne's desktop package and our
platform/desktop package by aliasing Fyne imports as fyneapp and
fynedesktop.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/app/version.go

@@ -0,0 +1,6 @@
+package app
+
+// Version is the application version shown in the GUI and used by build
+// scripts in artifact names. It is a var rather than a const so release builds
+// can override it with Go ldflags when CI tags a build.
+var Version = "0.3.2"