feat: lua pan/opacity, variables panel, monitor snap, icon fix

[konnatoad]

• SlideCommand: add pan_x, pan_y, opacity fields; parse_command reads them
• ViewerState: add lua_pan (Vec2) and lua_opacity (f32); apply in show_viewer as pixel offset + white-alpha tint on the image painter call
• app: call on_advance() after each slide advance; apply all lua fields (zoom, pan_x, pan_y, opacity) from both on_advance and on_interval
• app: monitor_snap field + first-frame SetWindowPos to force window onto preferred monitor, bypassing Windows "Remember window locations"
• lua_editor: Variables reference panel (toggle button in header) listing all ctx.* inputs and return { } outputs with types and descriptions; resize fix — desired_rows(4) + min_scrolled_height so window stays the size the user drags it to
• icon: re-apply blue #639BFF gradient background with white "ka" letterforms