feat: improve mobile experience (iOS Safari/Chrome)

[jglopez]

Tracking issue for mobile experience improvements, specifically iOS Safari and Chrome.

Two main areas:

1. Scroll behavior — users can over-scroll past the table bounds horizontally, exposing blank canvas. Needs to be constrained.
2. Narrow screen layout — on portrait phones, the table is cramped and hard to read. The goal is a stacked row layout where each row shows description/name on top and per-participant amounts on bottom, eliminating horizontal scrolling entirely.

Sub-issues cover the individual pieces: scroll fix, stacked layout, drag-to-reorder in the new layout, numeric keyboard for price inputs, touch target sizes, and a matching mobile layout for the summary section.