Serve the rulebook as a static HTML page from FastAPI

Replace the SPA Rules route with a self-contained src/pirats/rules.html
served at GET /rules (registered before the SPA mount, included in
package data for the Nix build). Plain anchor links replace the
scrollIntoView workaround since there's no hash router to fight. The
corner Rules link and splash-page link now point at /rules, and the
Vite dev server proxies /rules to the backend like /api.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/pirats/main.py

@@ -1,6 +1,7 @@
 from contextlib import asynccontextmanager
 from typing import Optional
 from fastapi import FastAPI, Form, Depends, HTTPException, APIRouter
+from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
 from sqlmodel import Session
 import uvicorn
@@ -105,6 +106,11 @@ api.include_router(admin_router)
 # Mount API at /api
 app.include_router(api, prefix="/api")
 
+# Static rulebook page (registered before the SPA mount so it takes precedence)
+@app.get("/rules", include_in_schema=False)
+def rules_page():
+    return FileResponse(BASE_DIR / "rules.html", media_type="text/html")
+
 # Mount SPA
 static_dir = BASE_DIR / "static"
 if os.path.exists(static_dir):