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.

.planning/ROADMAP.md

@@ -52,7 +52,7 @@ These rules govern every task. Any deviation must be discussed and documented as
 | 1.1  | Project scaffold (Vite + React + TS)                                                                         | 🟩     | (manual)  |
 | 1.2  | [Stack rounding-out (Tailwind + shadcn/ui + deps + Prettier)](./phase-1-foundation/02-stack-rounding-out.md) | 🟩     | `9918418` |
 | 1.3  | [Vite dev proxy + path aliases + tsconfig hardening](./phase-1-foundation/03-vite-dev-proxy.md)              | 🟩     | `39b60c9` |
-| 1.4  | [Runtime config endpoint](./phase-1-foundation/04-runtime-config.md)                                         | ⬜     | —         |
+| 1.4  | [Runtime config endpoint](./phase-1-foundation/04-runtime-config.md)                                         | 🟩     | `PENDING_SHA` |
 | 1.5  | [Directus auth client (cookie mode + refresh)](./phase-1-foundation/05-directus-auth-client.md)              | ⬜     | —         |
 | 1.6  | [Login page](./phase-1-foundation/06-login-page.md)                                                          | ⬜     | —         |
 | 1.7  | [Routing skeleton (TanStack Router + role-aware guards)](./phase-1-foundation/07-routing-skeleton.md)        | ⬜     | —         |