Rows with a trailing button (Folder / Choose…, watermark Graphic /
Choose…, Save preset / Save) sized their text field as the available
width minus a guessed button width. When the real button was wider
(e.g. Linux fonts, a folder picker present), the auto-sized dialog grew
a little every frame. These rows now lay out right to left: the button
first, then a field taking exactly the width that's left.
Test: export_dialog_keeps_its_width (grew 492 → 693 px in 60 frames
before; stable now).
Fixes#8
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>