Adds the universal "send with the platform modifier" shortcut every
mainstream mail client (Gmail, Outlook, Apple Mail, Proton, Tutanota,
Fastmail, Thunderbird) supports. Closes#343.
Behaviour:
* Window-level keydown listener registered while the composer is
mounted. Fires when focus is anywhere inside the composer — chip
inputs, subject, body textarea, or the rich-text contentEditable.
* Plain Enter is untouched; only Enter + Ctrl (Win/Linux) or Cmd
(macOS) triggers send. Shift/Alt modifiers are ignored so existing
autocomplete-confirm / chip-commit Enters are not hijacked.
* Routes through a ref so handleSend's per-render rebind doesn't
re-register the listener every render.
* All existing send-time validation, attachment-warning, draft-save
and undo-send flows still apply — the shortcut just calls the
same handleSend() as the toolbar button.
* Listed in the Keyboard Shortcuts dialog under the existing
Composer section.
Tested:
* Compose -> type body -> Ctrl+Enter -> Outbox.
* Cc/Bcc autocomplete suggestion + Enter still selects (alt-free
Enter without Ctrl, so the new listener bails).
* Subject input -> Ctrl+Enter -> sends.
* Body Enter without modifier -> newline.
