feat: redesign mode pills and dropdowns
Pills: extracted shared ModePill composable — thinner border (0.8dp gradient
instead of 1.2dp solid), subtler inactive background, smaller text (12.5sp),
compact padding (12x7 instead of 13x8). Active state uses accent gradient
border instead of solid color.
Reasoning dropdown: styled container (dark glass bg, 16dp shadow, squircle
shape), check icon on selected preset, cleaner row layout.
Sampling popover: same styled container, custom checkbox (squircle with accent
tint instead of Material Checkbox), disabled state dims labels and track,
wider panel (260dp), more spacing between parameters.
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