Style the DMG: dmgbuild layout, background, large icons

Layout is written by dmgbuild (Config/dmg/settings.py) instead of Finder
AppleScript, which proved unreliable. Background is generated by
scripts/make-dmg-background.swift.

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

CLAUDE.md

@@ -16,7 +16,7 @@ A native macOS app that shows video from a capture device (camera / capture card
   `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`, `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.
+- `scripts/check-strings.sh` – en/pl translation consistency. `scripts/make-dmg-background.swift` regenerates `Config/dmg/background.png`; keep its size and icon coordinates in sync with `Config/dmg/settings.py` (dmgbuild layout, installed into `build/venv` on first `make dmg`).
 - `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