chore: move gateway default port from 4000 to 14242

Port 4000 collides with too many dev tools (Phoenix, GraphQL tools, etc.).
Switch to 14242 — unregistered with IANA, no known conflicts, safely within
the User Ports range and outside Linux ephemeral port range (32768+).

Updates all hardcoded defaults across gateway, web client, CLI commands,
playwright config, .env.example, and docs. Bumps @mosaic/cli and
@mosaic/mosaic to 0.0.14.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/guides/dev-guide.md

@@ -112,11 +112,11 @@ DATABASE_URL=postgresql://mosaic:mosaic@localhost:5433/mosaic
 BETTER_AUTH_SECRET=change-me-to-a-random-secret
 
 # Gateway
-GATEWAY_PORT=4000
+GATEWAY_PORT=14242
 GATEWAY_CORS_ORIGIN=http://localhost:3000
 
 # Web
-NEXT_PUBLIC_GATEWAY_URL=http://localhost:4000
+NEXT_PUBLIC_GATEWAY_URL=http://localhost:14242
 
 # Optional: Ollama
 OLLAMA_BASE_URL=http://localhost:11434
@@ -141,7 +141,7 @@ migrations in production).
 pnpm --filter @mosaic/gateway exec tsx src/main.ts
 ```
 
-The gateway starts on port `4000` by default.
+The gateway starts on port `14242` by default.
 
 ### 6. Start the Web App
 
@@ -395,7 +395,7 @@ directory are defined there.
 
 ## API Endpoint Reference
 
-All endpoints are served by the gateway at `http://localhost:4000` by default.
+All endpoints are served by the gateway at `http://localhost:14242` by default.
 
 ### Authentication