Add Konverter: a local-only menu bar file converter

Drag files in Finder, hold a modifier, and drop on a wheel of outputs.
Conversions run entirely on-device through ImageIO, PDFKit, AVFoundation,
the bundled CLI tools, and optional ffmpeg/LibreOffice.

Includes a --self-test suite (41 checks) and a --convert CLI mode so the
conversion matrix can be verified without the UI.

.gitignore

@@ -0,0 +1,18 @@
+# SwiftPM
+.build/
+.swiftpm/
+
+# App bundle (build artifact, regenerate with ./build-app.sh)
+Konverter.app/
+
+# Secrets
+*.p8
+notary-result.json
+
+# macOS
+.DS_Store
+
+# Xcode
+xcuserdata/
+DerivedData/
+dist/