feat: add developer error page with request ID tracking and structured logging

Custom-built error handler for the web channel that shows a tabbed debug
dashboard (exception, request context, environment, SQL queries) in dev mode
and a clean branded error page with copyable request ID in prod mode.
All errors are logged as JSON lines to storage/logs/errors/ for lookup by
request ID. Uses the project's design tokens for visual consistency.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/Architecture/TypographyTokenContractTest.php

@@ -31,6 +31,7 @@ class TypographyTokenContractTest extends TestCase
         'web/css/vendor-overrides/',
         'web/css/base/variables.base.css',
         'web/css/base/typography.tokens.css',
+        'web/css/pages/app-error-debug.css', // Self-contained error page — inlined by ErrorRenderer, cannot use app tokens.
     ];
 
     /**