feat: tab bar

frontend/src/context/TabsContext.tsx

@@ -0,0 +1,27 @@
+import { createContext, useContext, useCallback, useState, ReactNode } from 'react'
+
+interface TabsCtx {
+  kept:        ReadonlySet<string>
+  keepRoute:   (r: string) => void
+  forgetRoute: (r: string) => void
+}
+
+const Ctx = createContext<TabsCtx>({
+  kept: new Set(),
+  keepRoute:   () => {},
+  forgetRoute: () => {},
+})
+
+export function TabsProvider({ children }: { children: ReactNode }) {
+  const [kept, setKept] = useState<Set<string>>(new Set())
+
+  const keepRoute = useCallback((r: string) =>
+    setKept(p => p.has(r) ? p : new Set([...p, r])), [])
+
+  const forgetRoute = useCallback((r: string) =>
+    setKept(p => { const n = new Set(p); n.delete(r); return n }), [])
+
+  return <Ctx.Provider value={{ kept, keepRoute, forgetRoute }}>{children}</Ctx.Provider>
+}
+
+export const useTabs = () => useContext(Ctx)