feat: add view-as-role feature for site admins (#15)

Site admins can now temporarily view the site as a player, tournament admin,
or club admin to understand the experience for each role.

Changes:
- Added RoleSwitcher context and provider for client-side role simulation
- Added role switcher dropdown in Navigation (visible only to site admins)
- Added yellow banner showing current viewing-as role with reset button
- Navigation links and wordmark href now use effective role for conditional display
- Added BDD feature file with 5 scenarios covering all role transitions
- Added step definitions for site admin login and role switcher interactions

The view-as feature is purely UI-level - server-side permissions remain unchanged.
5 scenarios, 27 steps, all passing.

src/components/RoleSwitcher.tsx

@@ -0,0 +1,37 @@
+"use client"
+
+import { createContext, useContext, useState, useCallback, ReactNode } from "react"
+
+type ViewAsRole = "player" | "tournament_admin" | "club_admin" | null
+
+interface RoleSwitcherContextType {
+  viewAsRole: ViewAsRole
+  setViewAsRole: (role: ViewAsRole) => void
+  effectiveRole: string | null
+}
+
+const RoleSwitcherContext = createContext<RoleSwitcherContextType | undefined>(undefined)
+
+export function RoleSwitcherProvider({ children }: { children: ReactNode }) {
+  const [viewAsRole, setViewAsRole] = useState<ViewAsRole>(null)
+
+  const value = {
+    viewAsRole,
+    setViewAsRole: useCallback((role: ViewAsRole) => setViewAsRole(role), []),
+    effectiveRole: viewAsRole,
+  }
+
+  return (
+    <RoleSwitcherContext.Provider value={value}>
+      {children}
+    </RoleSwitcherContext.Provider>
+  )
+}
+
+export function useRoleSwitcher() {
+  const context = useContext(RoleSwitcherContext)
+  if (!context) {
+    throw new Error("useRoleSwitcher must be used within RoleSwitcherProvider")
+  }
+  return context
+}