Allow applications to fix text field typography and icon button layout across themes #507

Ember NFR-L3 needs the same composer across systems. The field in EmberField specifies its fill, border, shape and height, but HostTextField derives typography and internal padding from theme.rules.field. HostButton applies theme button minimum size and padding even to an icon-only button with an explicit size. Switching the global theme changes the placeholder and plus button. Provide explicit field typography and content padding, and icon-button layout controls while preserving input and accessibility behavior. See DarkPyonix/darkpyonix-ember#739.