fix(app): allow Enter to send in mobile chat input (Shift+Enter for newline)

[synth-mania]

Note: This PR was written by pi-agent (the operator's personal AI assistant) at the operator's request, with the operator's review.

Summary

On mobile layouts (viewport ≤ 640px), Enter only inserts a newline and
sending requires Ctrl+Enter / Cmd+Enter (introduced in #472). With a
physical or Bluetooth keyboard attached to a phone, plain Enter appears
broken and the Ctrl/Meta+Enter chord is awkward to discover and hit.

This makes mobile behave like desktop: Enter sends, Shift+Enter inserts a
newline — the convention used by WhatsApp, Telegram, iMessage, Slack, etc.

Change

components/ChatInput.tsx — drop the mobile-only modifier requirement in
handleKeyDown:

-      const sendShortcut = e.key === "Enter" && !e.shiftKey && (!isMobile || e.ctrlKey || e.metaKey);
+      // Enter sends, Shift+Enter inserts a newline — same on every layout.
+      // Mobile previously required Ctrl/Meta+Enter (#472), which left
+      // hardware/Bluetooth keyboards with no way to send.
+      const sendShortcut = e.key === "Enter" && !e.shiftKey;

Notes / tradeoffs

• fix(app): allow Enter for newlines in mobile chat input #472 intentionally made Enter = newline on mobile for touch keyboards.
  With this change the on-screen keyboard's Return key sends (the pre-fix(app): allow Enter for newlines in mobile chat input #472
  send behavior); the send button remains for touch users.
• The old Ctrl+Enter / Cmd+Enter chord still works (it satisfies
  Enter && !shiftKey), so nothing existing stops working.
• The CJK IME guard (isComposing / keyCode 229 / 100 ms composition
  grace) is untouched, so Enter never sends mid-composition.
• On mobile, Enter now also confirms highlighted slash-command / @file /
  history menu items, consistent with desktop.
• isMobile remains in use for layout; no other behavior changes.

Test plan

• Mobile viewport + hardware keyboard: Enter sends; Shift+Enter newlines;
  Ctrl+Enter still sends.
• Desktop unchanged (Enter sends, Shift+Enter newlines).
• Slash / @file / history menus: Enter confirms the highlighted item on both
  layouts.
• tsc --noEmit clean. npm test: 586/587 pass locally; the single
  failure (lib/web-auth.test.mjs, password auth) reproduces on the clean
  tree, i.e. pre-existing and unrelated to this change.

[synth-mania]

Hey guys, it's me, the human behind the agent that made this PR.

I thought we should unify physical keyboard behavior across mobile and desktop platforms.

If you aren't comfortable with the manner in which this PR was authored, I would be happy to close the request.
Thanks!

[Ran-Xing]

I hope macOS will support option + return.

[huangyxi]

Hi, I’m the person who introduced the PR mentioned above. Sorry for the inconvenience.

However, if pressing Enter by itself is used to send the message, there’s no way for mobile users to start a new line in the mobile layout when no external keyboard is attached, since the default on-screen keyboards on both iOS and Android don’t send Shift+Enter when you press it.

Also, all major mobile LLM apps, including Claude, ChatGPT, and Gemini, use Enter to create a new line, while desktop apps use Enter to send the message (except Claude).

[agegr]

Thanks for the PR. We’re going to keep the mobile behavior introduced in #472: Enter inserts a newline, and the send button submits the message.

Mobile software keyboards generally have no Shift+Enter, so this change would make multiline input impossible. Since the browser cannot reliably distinguish software keyboards from external keyboards, we won’t merge this PR.