[Hackathon demo] Make the recording sheet adjustable with accessibility actions

Hackathon demo ticket. This is intentionally small and self-contained.

## Problem

The recording sheet grab area declares the adjustable role, but it does not handle accessibility increment or decrement actions. A screen-reader user cannot expand or collapse it through that role.

## Scope

Add the minimum accessibility actions and handler to the grab area in src/features/record/RecordSheet.tsx. Reuse the existing expanded state and spring behavior.

## Acceptance criteria

- Increment expands the sheet.
- Decrement collapses the sheet.
- The accessibility label reflects the current state.
- Touch pan and tap gestures continue to work.

## Validation

Follow docs/mobile-agent-loop.md.

Before: record the grab area's current attributes and show that the adjustable action does not change state.

After: use agent-device to invoke or verify the supported accessibility action, prove the expanded content becomes visible, and capture before and after screenshots.

The PR description must include clearly labeled Before and After evidence.

Read docs/mobile-agent-loop.md, then inspect the grab area in src/features/record/RecordSheet.tsx, including its current accessibility attributes, expanded state, and spring behavior. Use agent-device to verify increment and decrement actions, confirm the label and visible content reflect the state, check that touch gestures still work, and capture clearly labeled Before and After screenshots.

索引モデルが issue の本文から書いたものです。