Calendar: delete events with Delete/Backspace key and confirm with Enter

[PeterMcBTC]

Problem

Deleting an event in Proton Calendar currently requires the mouse twice:

1. Click the trash icon in the event popover.
2. Click the "Delete" button in the confirmation dialog.

Keyboard users have no way to do this: there is no shortcut to delete the
selected event, and pressing Enter in the delete confirmation dialog does not
confirm it. This is because Prompt deliberately excludes danger-colored
buttons from its Enter/Space submit hotkey (onSubmitHotkeyPress in
packages/components/components/prompt/Prompt.tsx), which is a sensible
default for destructive actions, but leaves no opt-in for flows where the
user explicitly wants keyboard-driven deletion.

Proposed solution

1. Delete/Backspace deletes the selected event. While an event popover is
   open (i.e. an event is selected), pressing Delete or Backspace triggers
   exactly the same flow as clicking the trash icon (handleDelete in
   EventPopover). The hotkey is registered only while the popover is
   mounted, so it cannot fire when no event is selected. It respects the
   "Keyboard shortcuts" mail setting and the existing isBusy() guard
   (no-op while typing in inputs, or while a modal/dropdown is open).

2. Enter confirms the deletion. A new opt-in prop
   enableDangerSubmitHotkey on Prompt allows the Enter hotkey to trigger
   the danger-colored CTA. It is enabled in DeleteConfirmModal and
   DeleteRecurringConfirmModal only — all other destructive prompts keep
   the current safe behavior. When enabled, only Enter is mapped; Space keeps
   its native behavior on the focused button, and preventDefault() avoids
   double-activation when the Cancel button is focused.

3. The new shortcut is documented in the keyboard shortcuts help modal
   (packages/shared/lib/shortcuts/calendar.ts).

Resulting flow: click event → Supr → Enter → event deleted.

Files changed

• packages/components/components/prompt/Prompt.tsx — new opt-in prop
  enableDangerSubmitHotkey (defaults to false, zero behavior change for
  existing consumers); the submit hotkey now calls preventDefault().
• applications/calendar/src/app/containers/calendar/confirmationModals/DeleteConfirmModal.tsx
  — enable Enter-to-confirm for single-event deletion.
• applications/calendar/src/app/containers/calendar/confirmationModals/DeleteRecurringConfirmModal.tsx
  — enable Enter-to-confirm for recurring-event deletion.
• applications/calendar/src/app/components/events/EventPopover.tsx —
  Delete/Backspace hotkey scoped to the open popover.
• packages/shared/lib/shortcuts/calendar.ts — document the new shortcut.

How to test

1. Enable keyboard shortcuts in Settings → General → Keyboard shortcuts.
2. Open the calendar, click an event to open its popover.
3. Press Delete (or Backspace) → the delete confirmation dialog opens.
4. Press Enter → the event is deleted.
5. Regression checks:
   • With the dialog open, Space on the focused Cancel button still cancels.
   • Delete/Backspace do nothing while typing in an input/textarea.
   • Other Prompt-based dialogs (without the new prop) behave as before.
   • Events in non-writable calendars (no trash button) are unaffected.