feat(web): allow send shortcuts in narrow windows

[jdbencardinop]

Request

Add an opt-in, persisted client setting that bypasses the web composer viewport-width submission guard. Keep it off by default so mobile software keyboards retain Enter-to-newline behavior.

A laptop user with a small screen and two apps side by side can have a web window at 639 CSS pixels or less while using a physical keyboard. Currently Enter inserts a newline, and Ctrl/Cmd+Enter cannot bypass the width guard; submitting the draft requires the arrow button.

Requested behavior

• When enabled, use the same send-shortcut rules as a normal-width window, without changing Shift+Enter, background draft submission, or existing Queue/Steer semantics.
• Make the setting discoverable, persisted, and reversible without resizing or reloading.
• Preserve existing behavior by default and leave normal-width windows unchanged.
• Integrate with upstream send-shortcut configuration rather than duplicating it.
• Keep automatic device/keyboard detection as a separate follow-up. Native mobile behavior and provider/server semantics are outside this request.

Where this came from

The width guard is an upstream mobile-web fix, not a fork-specific change. Issue #2478 reported that Android software keyboards could not create multiline prompts. PR #3930, merged July 17, introduced the mobile-width newline rule. PR #7821, merged August 21, added Ctrl/Cmd+Enter background submission but preserved that guard.

ChatComposer uses useMediaQuery("max-sm") as its mobile signal, and composerSubmissionIntentForEnter rejects keyboard submission before considering modifiers. Width therefore acts as a proxy for device/keyboard type. This request preserves the original mobile fix while offering a practical override for narrow laptop windows.

At investigation time on October 1, this checkout had no configurable send shortcut. Current upstream has Enter, Ctrl/Cmd+Enter, and Ctrl/Cmd+Enter-for-multiline modes, but still gives the mobile-width guard precedence.

Registered tpatch request

• Feature: composer-narrow-viewport-submit-override
• Lifecycle: requested, agent-authored through Path B; no implementation or phase advancement.
• Request branch, based on 982929e93.
• Full request, history, scope, and acceptance expectations.
• Lifecycle metadata.
• Registration commit: 8a5dd4140.

This issue tracks the proposal only; application code has not been changed.

Registered by GPT-6.1 Sol through GitHub Copilot CLI.