Custom global shortcuts for record and pause

[nftechie]

Context

Snapcap registers two system-wide shortcuts: ⌥⇧⌘R (start/stop) and ⌥⇧⌘P (pause/resume). They're hard-coded, so they can clash with other apps and some keyboard layouts. Settings already has a read-only "Keyboard shortcuts" section whose footer says custom shortcuts are on the roadmap.

GlobalHotKey wraps Carbon's RegisterEventHotKey, which needs no Accessibility permission. It takes a virtual key code plus Carbon modifier flags, returns nil when registration fails, and unregisters in deinit.

Where to look

• Sources/Snapcap/App/GlobalHotKey.swift: the wrapper.
• Sources/Snapcap/App/AppDelegate.swift: applicationDidFinishLaunching builds hotKeys with kVK_ANSI_R / kVK_ANSI_P and cmdKey | optionKey | shiftKey. makeMainMenu() hard-codes the same shortcuts on File → Start / Stop Recording and Pause / Resume Recording.
• Sources/Snapcap/App/StatusItemController.swift: buildMenu() sets the same key equivalents.
• Sources/Snapcap/Views/SettingsView.swift: the "Keyboard shortcuts" section (Shortcut view).
• Hard-coded hints: RecordBar in Sources/Snapcap/Views/RecorderView.swift, the footer of Sources/Snapcap/Views/QuickRecordView.swift, and the HUDButton help strings in Sources/Snapcap/Windows/RecordingPanels.swift.

Proposal

• Add a small Codable KeyboardShortcut-style value (key code + modifiers) that can produce its display string ("⌥⇧⌘R"), Carbon modifiers and NSEvent.ModifierFlags. Put the pure parts in SnapcapCore so they can be unit tested.
• In Settings, replace the read-only rows with a recorder control: click it, press a combination, done. Esc cancels, and there's a "Reset to default" button. Require at least one of ⌘, ⌥ or ⌃.
• Persist the shortcuts in UserDefaults, and re-register on change by dropping the old GlobalHotKey and creating a new one.
• If registration fails, show "This shortcut is in use by another app" and keep the previous one.

Acceptance criteria

• Both shortcuts can be changed in Settings and work immediately, without a relaunch.
• Choices persist across launches, and "Reset to default" restores ⌥⇧⌘R and ⌥⇧⌘P.
• A shortcut that can't be registered shows an error and never leaves the app without a working shortcut.
• The File menu, the menu bar menu, the setup window hint, the popover footer and the HUD tooltips all show the current shortcuts.
• Unit tests cover the display string and the modifier conversions.