Chat block: Enter sends, Shift+Enter inserts a newline

[lezama]

Summary

Standard chat-app keyboard handling on the input textarea:

• Enter → submits the form (same as clicking Send)
• Shift+Enter → inserts a newline (default textarea behavior)
• Modifier keys (Ctrl/Meta/Alt) and IME composing-state are excluded so dictation / accent picker / OS shortcuts don't accidentally submit

Verified

Synthesizing a real KeyboardEvent against the live block: `defaultPrevented: true`, transcript shows USER + the …thinking placeholder right after, REST POST goes out. Visual / UX-only change; no backend touched.

Test plan

• Type a message in the chat textarea, press Enter → message submits
• Type a message, press Shift+Enter → newline inserted, no submit
• Hold Cmd/Ctrl/Alt + Enter — does NOT submit (so OS shortcuts keep working)

Quality gate results

Gate	Result
php -l (no PHP changes)	n/a
vendor/bin/phpunit	✅ unchanged
tests/smoke.php	✅ unchanged
Browser test against live chat	✅