fix(launcher): ignore IME composition Enter so CJK input does not submit the search

[YuriNachos]

Fixes #637
(Also resolves its duplicate #296.)

What changed

Guard the main launcher search input's handleKeyDown against CJK IME
composition. Added a small pure predicate isImeComposing(nativeEvent, keyCode)
in src/renderer/src/utils/keyboard.ts and made it the first check in
handleKeyDown (src/renderer/src/hooks/useLauncherKeyboardControls.ts):
while an Input Method Editor composition is in progress (KeyboardEvent.isComposing === true,
or the legacy keyCode === 229), the handler returns early without
preventDefault, so the Enter that confirms a kana-kanji / pinyin candidate
is no longer treated as a submit.

Why

While composing Japanese/Chinese/Korean text, the Enter that confirms the IME
candidate arrives as a normal-looking Enter but with isComposing === true.
With no guard, that Enter hit case 'Enter': → e.preventDefault() → the
selected command ran / the window hid, mid-composition — making the launcher
unusable for CJK input. The sibling handler in QuickLinkManager.tsx:913
already guards with event.isComposing; the main search input was missing the
same guard. This brings it to parity via an extracted, unit-testable predicate.

Compatibility impact

No impact on Raycast extensions — this is launcher chrome (the main search
input's React keydown handler), not the @raycast/api / @raycast/utils
compatibility shim. Non-IME users see identical behavior: isImeComposing is
only true during an active composition (or a legacy 229 keydown), so no
ordinary keystroke is swallowed.

How tested

• New scripts/test-ime-composition.mjs (mirrors
  test-exec-command-timeout-cleanup.mjs, imports the real predicate via
  esbuild): 6 assertions, all pass — including
  isImeComposing({isComposing:true}, 13) === true,
  isImeComposing({}, 229) === true,
  isImeComposing({isComposing:false}, 13) === false.
• npm test → 88 tests, 87 pass, 0 fail.
• npm run build:main and npm run build:renderer (vite) → exit 0.
• npm run check:i18n → exit 0 (no strings changed).
• tsc -p tsconfig.renderer.json --noEmit delta vs main = 0 (the change
  adds no type errors; pre-existing renderer debt is unchanged).