feat: task 1.4 runtime config endpoint

Three-file config module under src/config/:
- schema.ts: RuntimeConfigSchema (directusUrl, liveWsUrl, businessWsUrl,
  optional googleMapsKey, env enum). Custom UrlOrAbsolutePath validator
  accepts http(s)/ws(s) URLs plus paths starting with /.
- load.ts: loadConfig() fetches /config.json with cache: 'no-store',
  safeParse, throws ConfigValidationError on schema failure.
- context.ts: RuntimeConfigContext + useRuntimeConfig() hook (no JSX).
- provider.tsx: RuntimeConfigProvider — loading / ready / error states.
  Single retry after 500ms on network failure; renders zod issues on
  validation failure for debuggability.

public/config.json: committed dev defaults (relative paths matching the
Vite proxy from 1.3).

src/main.tsx wraps App in <RuntimeConfigProvider>.
README gains a Runtime config section with the schema table.

Deviation: spec sketched provider+hook in one context.tsx. Split into
context.ts (hook only) and provider.tsx (component only) to satisfy
react-refresh/only-export-components without adding an eslint override.

public/config.json

@@ -0,0 +1,6 @@
+{
+  "directusUrl": "/api",
+  "liveWsUrl": "/ws-live",
+  "businessWsUrl": "/ws-business",
+  "env": "dev"
+}