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.