refactor(browser): split browser cores via build ARG, add core selector (#237)

* refactor(browser): split browser cores via build ARG, add core selector

- Replace playwright official image with ubuntu:noble base in both
  docker/Dockerfile.browser and devenv/Dockerfile.browser; install
  browsers at build time driven by ARG/ENV BROWSER_CORES
- Add GET /cores endpoint to Browser Gateway reporting available cores
- Proxy GET /browser-contexts/cores in Go handler to Browser Gateway
- Add `core` field to BrowserContextConfigModel and GatewayBrowserContext;
  context creation selects the appropriate browser instance by core
- Frontend context-setting page fetches available cores and renders a
  core selector; saves core as part of the config JSON
- install.sh prompts for browser core selection and writes BROWSER_CORES
  to .env; builds the browser image locally before docker compose up
- Regenerate OpenAPI spec and TypeScript SDK

* fix: lint

docker-compose.yml

@@ -118,8 +118,15 @@ services:
 
   browser:
     image: memohai/browser:latest
+    build:
+      context: .
+      dockerfile: docker/Dockerfile.browser
+      args:
+        BROWSER_CORES: ${BROWSER_CORES:-chromium,firefox}
     container_name: memoh-browser
     profiles: [browser]
+    environment:
+      - BROWSER_CORES=${BROWSER_CORES:-chromium,firefox}
     volumes:
       - ${MEMOH_CONFIG:-./config.toml}:/config.toml:ro
     ports: