fix: skip keyboard handler when focus is in a text input

Prevents useKeyboard from intercepting keypresses while the user
is typing in an input or textarea field, which caused wrong
characters to appear instead of Russian text.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useKeyboard.ts

@@ -17,6 +17,9 @@ export function useKeyboard(actions: CalculatorActions) {
 
   const handleKeyDown = useCallback(
     (e: KeyboardEvent) => {
+      const tag = (e.target as HTMLElement)?.tagName;
+      if (tag === 'INPUT' || tag === 'TEXTAREA') return;
+
       const mapped = KEY_MAP[e.key];
       if (!mapped) return;