Drive the composer's layout off its OWN measured width (the existing
ResizeObserver, so it reacts per-tile, not per-viewport) instead of only
stacking or overflowing:
- >=440px: full inline row, full model label.
- 320-440px: still inline, but the model pill sheds its label for its chevron
icon (~120px back) so the controls stop crowding the placeholder.
- <320px: stacks to the two-row layout, pill stays iconized.
Adds COMPOSER_COMPACT_PILL_PX above the stack breakpoint and a `compactPill`
signal from useComposerMetrics, wired to the model pill's existing compact mode.
305e26558e