docs: add React Router URL routing design spec

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

docs/superpowers/specs/2026-04-24-react-router-url-routing-design.md

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+# React Router URL Routing — Design Spec
+
+**Date:** 2026-04-24
+**Status:** Approved
+
+## Overview
+
+Add React Router v7 to the Brainwave frontend so the URL reflects the active exam and current question number. A user on question 3 of "My Exam" will see `/exam/my-exam/q3` in the address bar. Back/forward browser navigation works naturally, and deep links to a specific question are supported.
+
+## Route Structure
+
+| URL | Screen |
+| --- | ------ |
+| `/` | Landing |
+| `/exam/:slug/q:n` | ExamRunner — question `n` (1-indexed) of exam identified by `slug` |
+| `/results` | Results (post-submission) |
+| `/review/:slug` | Results in review-only mode |
+| `/verify` | Email verification |
+| `/reset-password` | Password reset |
+| `/admin` | Admin dashboard (admin users only) |
+
+`:slug` is the exam title converted to lowercase kebab-case (e.g. `"My Exam 2024"` → `"my-exam-2024"`). `:n` is the 1-indexed question number.
+
+## Architecture
+
+### New dependency
+
+`react-router-dom` v7 added to `frontend/package.json` dependencies.
+
+### New utility — `frontend/src/utils/slug.ts`
+
+Pure function `slugify(title: string): string`:
+
+- Lowercase
+- Replace non-alphanumeric characters with hyphens
+- Collapse consecutive hyphens
+- Trim leading/trailing hyphens
+
+### `main.tsx`
+
+Wrap `<App>` in `<BrowserRouter>`.
+
+### `App.tsx`
+
+- Remove `screen` state and `AppScreen` type usage.
+- Replace the URL-on-mount `useEffect` with React Router's declarative routing.
+- Use `<Routes>` / `<Route>` to render screen components.
+- Replace all `setScreen()` calls with `useNavigate()`:
+  - `loadExam()` → `navigate('/exam/${slug}/q1')`
+  - `submit()` → `navigate('/results')`
+  - `exit()` → `navigate('/')`
+  - `reviewAttempt()` → `navigate('/review/${slug}')`
+  - Admin menu item → `navigate('/admin')`
+  - `handleAuthFlowDone()` → `navigate('/')`
+- `exam` and `examState` remain in React state — no change to storage, grading, or submission logic.
+
+### `ExamRunner.tsx`
+
+- Accept an `onNavigate: (i: number) => void` prop, called from `goTo()` after updating `examState`.
+- `App.tsx` passes a handler that calls `navigate('/exam/${slug}/q${i+1}')`.
+- ExamRunner has no direct knowledge of React Router — it remains router-agnostic.
+
+### Exam route handler in `App.tsx`
+
+- Reads `:n` and `:slug` from `useParams()`.
+- On mount and whenever `:n` changes (back/forward), calls `setExamState` to set `currentIdx = n - 1`.
+- Handles the deep-link case: if no active exam in state, checks localStorage for a matching slug; resumes or redirects to `/`.
+
+### Deep linking
+
+When the app loads on `/exam/:slug/q:n`:
+
+- Check localStorage for a saved exam state (`brainwave.exam.state`).
+- If the saved exam's slugified title matches `:slug`, resume with `currentIdx = n - 1`.
+- If no match, redirect to `/`.
+
+### Vite config
+
+Add `server.historyApiFallback: true` to `vite.config.ts` so the dev server serves `index.html` for all non-asset paths. The production backend already handles this (it serves `index.html` for non-API routes).
+
+## Data Flow
+
+```text
+User clicks question 5 in sidebar
+  → goTo(4) called in ExamRunner
+  → setState updates examState.currentIdx = 4
+  → onNavigate(4) called
+  → navigate('/exam/my-exam/q5') in App
+  → URL bar updates
+  → :n param changes
+  → useEffect in App exam route calls setExamState [no-op, already 4]
+
+User presses browser Back
+  → URL changes to /exam/my-exam/q4
+  → :n param changes to '4'
+  → useEffect in App exam route calls setExamState({ currentIdx: 3 })
+  → ExamRunner re-renders at question 4
+```
+
+## What Does Not Change
+
+- `Results.tsx`, `Landing.tsx`, all question components
+- `AuthContext`, storage utils, grading utils, API utils
+- Backend — no changes required
+- `ExamState` shape and localStorage persistence
+- `AppScreen` type removed from `types.ts`
+
+## Out of Scope
+
+- Animated route transitions
+- Scroll restoration between questions
+- Sharing/bookmarking exam links to others (exam data is client-side only)