docs: add exception for inline CSS in error template

Allow inline CSS in templates/error.templ since error pages must work
when the main app fails (404, server errors, CSS fails to load).

PROJECT_GUIDELINES.md

@@ -26,12 +26,13 @@
 ### Frontend & Styling
 - ❌ **NEVER modify backend/API for frontend features without user confirmation**
 - ❌ **NEVER use custom CSS** - TailwindCSS classes only
+  - **⚠️ EXCEPTION**: `templates/error.templ` may have inline CSS because error pages must work when main app fails (404, server errors, CSS fails to load)
 - ❌ **NEVER use JavaScript** - convert all to TypeScript
 - ❌ **NEVER use Object-Oriented Programming** (no classes, inheritance, or this-capture)
 - ✅ **DO use procedural/imperative style** as your default
 - ✅ **DO borrow functional techniques** when they simplify code
 - ✅ **DO avoid ideological purity** - the best paradigm is the one that fits the problem
-- ❌ **NEVER add new Dockerfiles without user confirmation
+- ❌ **NEVER add new Dockerfiles without user confirmation**
 - ❌ **NEVER fetch initial data via AJAX on page load** - use server-side rendering instead
 - ❌ **NEVER break progressive enhancement** - pages must work without JavaScript