Add webuiHost / webuiTls.enabled / trustProxy to RuntimeConfig as the shared
system-settings model for WebUI Wave A1 (HTTPS + self-config panel). Refactor
load into pure, tested normalizeRuntimeConfig + resolveRuntimeEnvOverrides
(env wins, in-memory only; SNOWLUMA_WEBUI_HOST added alongside the existing
PORT/TRUST_PROXY overrides). loadRuntimeConfig backfills older runtime.json
with the new defaults. Adds vitest to @snowluma/common (was typecheck-only).