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>

packages/auth/src/auth.ts

@@ -35,7 +35,7 @@ export function createAuth(config: AuthConfig) {
       provider: 'pg',
       usePlural: true,
     }),
-    baseURL: baseURL ?? process.env['BETTER_AUTH_URL'] ?? 'http://localhost:4000',
+    baseURL: baseURL ?? process.env['BETTER_AUTH_URL'] ?? 'http://localhost:14242',
     secret: secret ?? process.env['BETTER_AUTH_SECRET'],
     basePath: '/api/auth',
     trustedOrigins,