feat: themed 404 and 500 pages

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

fabledscryer/app.py

@@ -2,7 +2,7 @@
 from __future__ import annotations
 import asyncio
 from pathlib import Path
-from quart import Quart
+from quart import Quart, render_template
 from .config import load_bootstrap
 from .database import init_db
 
@@ -161,6 +161,15 @@ def create_app(
     async def health():
         return {"status": "ok"}
 
+    # ── 12. Error handlers ─────────────────────────────────────────────────────
+    @app.errorhandler(404)
+    async def not_found(_):
+        return await render_template("errors/404.html"), 404
+
+    @app.errorhandler(500)
+    async def server_error(_):
+        return await render_template("errors/500.html"), 500
+
     # ── 11. Startup hook ───────────────────────────────────────────────────────
     @app.before_serving
     async def startup():