Localization scaffolding (String Catalog, no hard-coded UI strings)

[nftechie]

Context

All UI text is written as English string literals. We'd love translations, but first the app needs to be localizable. This issue covers the scaffolding and moving every user-facing string into a catalog, not the translations themselves.

Where strings live

• SwiftUI views in Sources/Snapcap/Views/ (including SettingsView.swift, OnboardingView.swift, QuickRecordView.swift and EditorView.swift) and in Sources/Snapcap/Windows/RecordingPanels.swift. Literals passed to Text, Button and Toggle are already LocalizedStringKeys, but strings passed as String aren't. Examples: SliderRow(title:), Banner(title:message:), HUDButton(help:), and EditorModel.showToast(_:) messages.
• AppKit strings: NSMenuItem titles in Sources/Snapcap/App/AppDelegate.swift (makeMainMenu) and StatusItemController.swift (buildMenu), plus NSAlert text in AppModel.swift, AppDelegate.swift and LibraryView.swift.
• SnapcapCore labels shown in the UI: CameraShape.label, CameraAspect.label, ExportResolution.label, ExportCodec.label and RecorderError.errorDescription.
• Sources/Snapcap/Windows/RegionSelector.swift, which draws text with NSString.draw.
• Resources/Info.plist: NSCameraUsageDescription and NSMicrophoneUsageDescription, which need an InfoPlist.strings.
• Build: Package.swift has no defaultLocalization or resources yet, and scripts/build-app.sh assembles Snapcap.app by hand.

Proposal

• Add a String Catalog (Localizable.xcstrings) with English as the development language. SwiftUI and String(localized:) look in Bundle.main by default, so the simplest route is to compile the catalog into en.lproj inside Snapcap.app/Contents/Resources during build-app.sh (for example with xcrun xcstringstool compile). The alternative is SwiftPM resources with bundle: .module everywhere. Pick one and document it.
• Convert String parameters to LocalizedStringResource or String(localized:). Use plural variations for counts, such as "1 recording" and "2 recordings" in LibraryView.
• Add CFBundleLocalizations to Info.plist.

Acceptance criteria

• Every user-facing string (SwiftUI, AppKit menus and alerts, toasts, core labels and errors, permission prompts) is in the catalog.
• Launching with build/Snapcap.app/Contents/MacOS/Snapcap -NSDoubleLocalizedStrings YES doubles every visible string in the setup window, HUD, countdown, editor, Recordings, Settings, onboarding, popover and menus, which shows nothing is left hard-coded.
• Plurals are handled correctly.
• make run and make test still work, and the English UI is unchanged.
• CONTRIBUTING.md gains a short "Adding a translation" section.