feat(rules): add URL params for direct links to board rules

Support /rules/:boardIdentifier (e.g. /rules/pol, /rules/somedomain.eth).
Selection updates URL; rules page always uses yotsuba theme.

src/app.tsx

@@ -194,7 +194,7 @@ const App = () => (
       <Route element={<GlobalLayout />}>
         <Route path='/' element={<Home />} />
         <Route path='/faq' element={<FAQ />} />
-        <Route path='/rules' element={<Rules />} />
+        <Route path='/rules/:boardIdentifier?' element={<Rules />} />
         <Route element={<BoardLayout />}>
           <Route path='/all/:timeFilterName?' element={null} />
           <Route path='/all/:timeFilterName?/settings' element={null} />