fix(word-addin): scope Escape to the open dropdown inside modals

WHY THIS MATTERS
Pressing Escape with a select dropdown open inside the workflow modals
closed BOTH the dropdown and the modal, discarding everything typed
into the form. Escape should peel one layer at a time: first the
dropdown, then (on a second press) the modal.

WHAT IS THE EVENT-ORDER MECHANISM
Radix's dismissable layer registers a capture-phase keydown listener on
document; the Modal listens bubble-phase on window. Capture on document
runs before the event bubbles back out to window, so stopping
propagation inside the Radix handler kills the event before the Modal
ever sees it -- no timing hacks, just DOM event phases.

HOW IT WORKS
ModalSelect passes onEscapeKeyDown={(e) => e.stopPropagation()} to its
DropdownContent. The dropdown still closes (no preventDefault), and a
second Escape -- with the Radix layer unmounted -- bubbles to window
and closes the modal as before. The behavior is opt-in at the
ModalSelect call site rather than baked into the shared Dropdown
primitive, because that primitive also serves non-modal surfaces
(header menu, history, document source, model toggle) where swallowing
Escape could collide with the prompt editor's document-level handler.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015U1iPxsADQ2yuksNmDuvvW