fix(terminal): swallow Shift+Enter keypress so it no longer submits

xterm runs the custom key handler for keypress too and drops Ctrl/Alt
keypresses but not Shift-only ones, so the stray \r submitted the prompt
after the newline. Swallow every event type for Shift/Ctrl+Enter and send
only on keydown, in the primary pane and Pane B. Adds a static guard and a
real xterm + Chromium browser test.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

test/shift-enter-keypress-swallowed.test.ts

@@ -0,0 +1,21 @@
+// @vitest-environment node
+// Regression guard: xterm runs the custom key handler for keydown AND keypress.
+// It discards a keypress carrying Ctrl/Alt but not one carrying only Shift, so
+// a handler that returns false for keydown alone lets Shift+Enter's keypress
+// through as a bare \r (submit). The Enter gate must therefore not be keyed on
+// ev.type === 'keydown'; only the send-key fetch is.
+
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
+import { describe, expect, it } from 'vitest';
+
+const PUBLIC = join(new URL('.', import.meta.url).pathname, '../src/web/public');
+
+describe.each(['terminal-ui.js', 'terminal-split.js'])('%s Shift/Ctrl+Enter handler', (file) => {
+  const src = readFileSync(join(PUBLIC, file), 'utf8');
+
+  it('swallows every event type for Shift/Ctrl+Enter', () => {
+    expect(src).toMatch(/ev\.key === 'Enter' && \(ev\.shiftKey \|\| ev\.ctrlKey\)\) \{/);
+    expect(src).not.toMatch(/ev\.key === 'Enter' && \(ev\.shiftKey \|\| ev\.ctrlKey\) && ev\.type === 'keydown'/);
+  });
+});