fix: prevent hidden textarea focus from scrolling page

Summary

• keep the hidden textarea focus target fixed within the viewport instead of positioning it far off-screen
• use focus({ preventScroll: true }) for the paste shortcut refocus path

This prevents browsers from scrolling the page toward the hidden input while typing or pasting.

Verification

• corepack pnpm install --frozen-lockfile
• corepack pnpm --filter @wterm/core build
• corepack pnpm --filter @wterm/dom type-check
• corepack pnpm --filter @wterm/dom test

DOM package tests: 68 passed.