feat: stacked row layout for narrow screens

[jglopez]

Problem

On narrow portrait-mode phones, the full-width table is crammed into a horizontal scroll view. Reading item names and participant amounts at the same time is awkward.

Proposed solution

Below the sm breakpoint (640px), transform each table row into a two-line card:

• Top: item description and name
• Bottom: per-participant amounts

This eliminates horizontal scrolling on narrow screens entirely.

Notes

• Only activate below sm (640px). Desktop and tablet layouts are unchanged.
• Participant count is dynamic (1–N), so the bottom section must handle any number gracefully.
• Main file: src/components/ItemsTable.tsx

Acceptance criteria

• On a portrait-width phone, each row renders as a stacked card.
• No horizontal scrolling needed to see all amounts.
• Layout switches back to normal table above 640px.