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

devenv/Dockerfile.browser

@@ -1,7 +1,17 @@
-FROM mcr.microsoft.com/playwright:v1.50.0-noble
+FROM ubuntu:noble
+
+ARG BROWSER_CORES=chromium,firefox
+ENV BROWSER_CORES=$BROWSER_CORES
+
+RUN apt-get update && apt-get install -y --no-install-recommends unzip curl nodejs npm && \
+    curl -fsSL https://bun.sh/install | bash && \
+    npm install -g playwright@1.50.0 && \
+    for core in $(echo "$BROWSER_CORES" | tr ',' ' '); do \
+      npx playwright install --with-deps "$core"; \
+    done && \
+    npm uninstall -g playwright && \
+    apt-get clean && rm -rf /var/lib/apt/lists/*
 
-RUN apt-get update && apt-get install -y --no-install-recommends unzip && rm -rf /var/lib/apt/lists/*
-RUN curl -fsSL https://bun.sh/install | bash
 ENV PATH="/root/.bun/bin:${PATH}"
 
 WORKDIR /workspace