Add IsMobile property and BoolToTabPlacementConverter; wire
TabStripPlacement to move tabs to the bottom on mobile (standard
Android pattern). Hide the desktop menu bar on mobile and expose
Settings/Help via a compact header row instead.
Replace fixed-width multi-column grids with dual desktop/mobile
templates in History, Addresses and Contacts lists; adapt Send,
Receive and the add-contact form for narrow screens. Remove hard-coded
Width from all five overlays (560–640 px) in favour of MaxWidth +
Margin="16" so they never overflow a 360 px screen; add ScrollViewer
to Address info, Server settings and Settings overlays.
Desktop layout is unchanged: all differences are gated on the
IsMobile/IsDesktop bool, which is a platform constant (false on desktop).
