feat(composer): Ctrl+Enter / Cmd+Enter sends the open draft

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.

components/email/email-composer.tsx

@@ -854,6 +854,7 @@ export function EmailComposer({
     return () => window.removeEventListener('keydown', handleTemplateKey);
   }, []);
 
+
   const addFiles = useCallback(async (files: File[]) => {
     if (!client || files.length === 0) return;
 
@@ -1574,6 +1575,28 @@ export function EmailComposer({
     }
   };
 
+  // Ctrl+Enter (Windows/Linux) / Cmd+Enter (macOS) sends the open
+  // compose draft — same as every other major mail client. Fires
+  // even when focus is inside the To/Cc/Bcc chips, subject input,
+  // body textarea, or the rich-text editor's contentEditable region.
+  // Plain Enter in the body still inserts a newline; only Enter +
+  // the platform modifier sends. handleSend is rebound every render,
+  // so we route through a ref to keep the window listener stable.
+  const handleSendRef = useRef<(skipAttachmentCheck?: boolean) => Promise<void>>();
+  handleSendRef.current = handleSend;
+  useEffect(() => {
+    const handleSendShortcut = (e: KeyboardEvent) => {
+      if (e.key !== 'Enter') return;
+      if (!(e.ctrlKey || e.metaKey)) return;
+      // Don't hijack autocomplete-confirm or chip-commit Enters.
+      if (e.altKey || e.shiftKey) return;
+      e.preventDefault();
+      void handleSendRef.current?.();
+    };
+    window.addEventListener('keydown', handleSendShortcut);
+    return () => window.removeEventListener('keydown', handleSendShortcut);
+  }, []);
+
   const cleanClose = () => {
     if (saveTimeoutRef.current) {
       clearTimeout(saveTimeoutRef.current);