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>

scripts/build-linux.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 
 # Optional first argument lets a developer or CI job choose the output path. The
 # default includes the application version and target platform.
-version="$(sed -n 's/^var Version = "\(.*\)"/\1/p' src/core/version.go)"
+version="$(sed -n 's/^var Version = "\(.*\)"/\1/p' src/app/version.go)"
 version="${version:-0.0.0-dev}"
 output="${1:-dist/linux/gosentry-${version}-linux-amd64}"
 mkdir -p "$(dirname "$output")"
@@ -17,7 +17,7 @@ export GOARCH=amd64
 
 # -trimpath removes local machine paths from debug/build metadata. -s -w strips
 # symbol/debug tables to keep the desktop binary smaller.
-go build -trimpath -ldflags "-s -w -X gitea.mixdep.ru/mix/gosentry/src/core.Version=${version}" -o "$output" ./cmd/gosentry
+go build -trimpath -ldflags "-s -w -X gitea.mixdep.ru/mix/gosentry/src/app.Version=${version}" -o "$output" ./cmd/gosentry
 
 # The application icon is embedded by Go, so the Linux build does not need a
 # sidecar assets directory beside the executable.