Add preview window and controls

new "iv Preview" auxiliary window and related UI helpers to control image preview adjustments. Adds a new offset parameter and a linear_interpolation toggle to PlaceholderUiState, PreviewControls and PreviewPushConstants, persists/parses/saves the offset, and clamps it to [-1,1]. Update the Vulkan shader to apply the offset to RGB and to support nearest/linear sampling based on the linear_interpolation flag. Add helper functions for consistent aux window defaults, preview button styling, mode handlers, and layout registration; add a menu item and env flags to show the preview window. Wire the controls into preview texture updates and push constants so the preview reflects exposure, gamma, offset, color modes and interpolation settings.

Author: ssh4net