fix/194 Add global Escape-to-close for custom modals

[michelroegl-brunner]

Implements a global ESC-to-close behavior across custom modals.

Key points:

• Adds ModalStackProvider with a single keydown listener to close the topmost eligible modal.
• Allows ESC even when typing in inputs (per user request).
• Protects destructive/critical flows: danger ConfirmationModal, AuthModal, LoadingModal are not dismissible via ESC.
• Wires all modals to register/unregister with the stack, providing allowEscape and onClose.
• Wrapped app in provider in src/app/layout.tsx.
• Fixed lint issues and ensured successful build (npm run build passes).

Testing:

• Open any non-critical modal and press ESC: closes.
• Focus an input inside any non-critical modal and press ESC: closes.
• Open danger confirmation, auth or loading modals and press ESC: no effect.
• When multiple modals open, ESC only closes the topmost eligible modal.