Fix terminal right edge readability

[ovdmar]

Summary

• reserve a visible right gutter in the xterm surface so right-edge output remains readable
• resize xterm/PTY using proposed fit dimensions minus reserved readable columns
• add unit and desktop E2E coverage for right-edge terminal output

Verification

• pnpm vitest run apps/web/src/terminal-pane-resize.test.ts apps/web/src/terminal-pane.test.ts apps/web/src/terminal-pane-basic.test.ts apps/web/src/terminal-pane-disconnect.test.ts apps/web/src/terminal-pane-wheel.test.ts
• pnpm typecheck
• pnpm lint
• pnpm build
• pnpm e2e
• pnpm exec playwright test e2e/operator-cockpit.spec.ts --project=desktop --grep "desktop terminal surface"