fix(agents-server-ui): mobile composer submit (Send button + soft-keyboard Enter)

[nharsch]

Fixes #4401.

Problem

The agent UI composer at /__agent_ui/ silently dropped message submissions on mobile browsers — both Send button taps and virtual-keyboard Enter were non-functional. The textarea accepted text but nothing was ever sent.

Two distinct root causes, both in packages/agents-server-ui/src/components/MessageInput.tsx:

Send button. Tapping the button blurred the textarea, the on-screen keyboard began dismissing, and the viewport reflowed between pointerdown and pointerup. The resulting click landed off the button and the send never fired.

Virtual-keyboard Enter. The keydown handler only matched e.key === 'Enter', but Android Chrome / GBoard frequently report key === 'Unidentified' / keyCode === 229 (IME-coordinated path) or route the return key through beforeinput as insertLineBreak with no matching keydown at all.

Fix

• enterKeyHint="send" on the textarea so soft keyboards surface a send-shaped action key and (critically on Android Chrome) fire keydown with key === 'Enter' reliably.
• IME guard in the keydown handler — skip submit while e.nativeEvent.isComposing is true, and explicitly skip e.keyCode === 229 (Android Chrome doesn't always set isComposing).
• onBeforeInput fallback matching inputType === 'insertLineBreak' for soft keyboards that route the return key through beforeinput without a matching keydown.
• onPointerDown preventDefault for non-mouse pointers on the Send button — keeps focus on the textarea so the on-screen keyboard doesn't dismiss, the viewport doesn't reflow between pointerdown and pointerup, and the resulting click lands on the button as intended. Mouse pointers are excluded so desktop click behaviour is unchanged.

Tested on

• ✅ Android Chrome — Send button and virtual-keyboard Enter both submit
• ✅ iOS Chrome — same
• ✅ Desktop (Linux Chrome) — Enter and click still work; no regressions

Verified locally against the bundled UI in electricax/agents-server:latest with packages/agents-server-ui/dist volume-mounted in.

Changeset

A patch-level @electric-ax/agents-server-ui changeset is included (.changeset/agent-ui-mobile-submit.md).