chore: replace volta with mise for toolchain management

chore: replace volta with mise for toolchain management

Volta is unmaintained and its maintainers recommend mise. Declare the
node and pnpm toolchain through mise, reading versions from package.json
(via devEngines.runtime and packageManager), drop the volta block, and
ignore personal mise overrides and lockfiles.

CI is unchanged: it already provisions node and pnpm through
actions/setup-node and pnpm/action-setup.

Mirrors SAP/e-mobility-charging-stations-simulator commit 8b5bda42
(chore: replace volta with mise for toolchain management).