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

spec/docs.go

@@ -5482,6 +5482,32 @@ const docTemplate = `{
                 }
             }
         },
+        "/browser-contexts/cores": {
+            "get": {
+                "description": "Get the list of browser cores available in the Browser Gateway container",
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "browser-contexts"
+                ],
+                "summary": "Get available browser cores",
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/handlers.BrowserCoresResponse"
+                        }
+                    },
+                    "502": {
+                        "description": "Bad Gateway",
+                        "schema": {
+                            "$ref": "#/definitions/handlers.ErrorResponse"
+                        }
+                    }
+                }
+            }
+        },
         "/browser-contexts/{id}": {
             "get": {
                 "description": "Get browser context by ID",
@@ -10039,6 +10065,17 @@ const docTemplate = `{
                 }
             }
         },
+        "handlers.BrowserCoresResponse": {
+            "type": "object",
+            "properties": {
+                "cores": {
+                    "type": "array",
+                    "items": {
+                        "type": "string"
+                    }
+                }
+            }
+        },
         "handlers.ChannelMeta": {
             "type": "object",
             "properties": {