feat(desktop): support multiline chat input

[henrynguyeninfo1]

Summary

• make Shift+Enter insert an explicit newline in the Desktop chat composer while keeping Enter-to-send as the default
• flush the contenteditable draft immediately so multiline content is preserved when sent
• add composer regression coverage for Shift+Enter newline insertion and multiline submit payloads

Fixes #52952.

Test plan

• npm --workspace apps/desktop run test:ui -- src/app/chat/composer/rich-editor.test.ts src/app/chat/composer/enter-submit-dom-race.test.tsx
• npm --workspace apps/desktop exec eslint src/app/chat/composer/index.tsx src/app/chat/composer/enter-submit-dom-race.test.tsx
• npm --workspace apps/desktop run typecheck

RISK: GUI behavior was not manually verified in a packaged Electron window; validation is automated component/unit coverage plus typecheck/lint.

[tonydwb]

Code Review Summary

Verdict: Approved

Multiline chat input feature for desktop. Clean, simple addition. No security concerns.

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Reviewed by Hermes Agent

[teknium1]

Thanks for the focused Desktop regression coverage. This is an automated hermes-sweeper review; current main already provides the requested behavior, so this PR is redundant.

• apps/desktop/src/app/chat/composer/index.tsx:577 submits only when !event.shiftKey, leaving Shift+Enter to insert the contenteditable newline.
• apps/desktop/src/app/chat/composer/hooks/use-composer-submit.ts:118-134 synchronously reads the live editor DOM before submitting, preserving multiline content despite deferred input flushing.
• apps/desktop/src/lib/keybinds/actions.ts:196-200 and apps/desktop/src/i18n/en.ts:1745 already advertise Shift+Enter as the composer newline shortcut.
• apps/desktop/src/app/chat/composer/rich-editor.ts:322-363 explicitly preserves real Shift+Enter line breaks during editor normalization.