Add make dmg with install instructions; build universal binary

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -14,10 +14,10 @@ A native macOS app that shows video from a capture device (camera / capture card
 ## Structure
 - `Sources/` – app code (AppKit + AVFoundation, one `AVCaptureSession`, audio via `AVCaptureAudioPreviewOutput`). The `App` class is split into extensions:
   `main.swift` (entry point), `App.swift` (state, window, lifecycle), `Capture.swift` (devices, formats, hot-plug), `Menu.swift` (menus and actions), `Power.swift` (sleep prevention), `Preferences.swift` (UserDefaults-backed settings).
-- `Config/Info.plist`, `Config/SecondDisplay.entitlements` – version, permissions (camera, microphone; hardened runtime).
+- `Config/Info.plist`, `Config/SecondDisplay.entitlements`, `Config/dmg/` – version, permissions (camera, microphone; hardened runtime).
 - `Resources/` – `AppIcon.icon` (Icon Composer, Liquid Glass; compiled by `actool`), `{en,pl}.lproj` (`Localizable.strings`, `InfoPlist.strings`), `uninstall.sh` (copied into the bundle; removes the app, preferences, caches, TCC permissions). Bundle ID: `com.mmacierzanka.seconddisplay`.
 - `scripts/check-strings.sh` – en/pl translation consistency.
-- `Makefile` – `make` (builds `build/SecondDisplay.app`), `run`, `check`, `install`, `uninstall`, `clean`, `help`. `SIGN_ID` defaults to ad-hoc (`-`).
+- `Makefile` – `make` (builds `build/SecondDisplay.app`), `run`, `check`, `install`, `uninstall`, `dmg`, `clean`, `help`. Builds are universal (arm64 + x86_64, macOS 26+). `make dmg` makes an un-notarized DMG with `Config/dmg/Read Me First.txt`. `SIGN_ID` defaults to ad-hoc (`-`).
 
 ## Conventions
 - Settings live in `UserDefaults` (keys: `video`, `audio`, `keepAwake`, `startFull`, `onTop`, `gravity`, `format.<id>`).