docs: add development standards to prevent navigation anti-patterns

- Document proper Next.js router usage vs window.location.href
- Establish clear rules for navigation and fullscreen state management
- Include code review checklist and migration guide
- Prevent future instances of page reload navigation mistakes
- Emphasize performance and state preservation benefits

Key rules:
- ALWAYS use useRouter().push() for navigation
- NEVER use window.location.href (breaks state, exits fullscreen)
- Manual fullscreen entry only (respect browser security)
- Let client-side navigation preserve fullscreen automatically

🤖 Generated with [Claude Code](https://claude.ai/code)

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

apps/web/DEVELOPMENT_STANDARDS.md

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+# Development Standards - Soroban Abacus Web App
+
+## 🚨 Critical Rules - DO NOT BREAK THESE
+
+### Navigation Standards
+
+#### ✅ ALWAYS Use Next.js Router for Navigation
+```typescript
+// ✅ CORRECT - Use Next.js router
+import { useRouter } from 'next/navigation'
+
+const router = useRouter()
+router.push('/target-route')
+```
+
+#### ❌ NEVER Use window.location.href
+```typescript
+// ❌ WRONG - Causes page reloads, breaks state, kills fullscreen
+window.location.href = '/target-route'
+```
+
+**Why this matters:**
+- `window.location.href` causes full page reloads
+- Page reloads break React state, exit fullscreen mode, and hurt performance
+- Next.js router provides client-side navigation that preserves state
+- **This is literally the main reason we use Next.js!**
+
+### Fullscreen State Management
+
+#### ✅ Let Client-Side Navigation Preserve Fullscreen
+- Users enter fullscreen manually (respects browser security)
+- Navigation preserves fullscreen automatically via client-side routing
+- No need for complex restoration logic
+
+#### ❌ Never Try to Auto-Enter Fullscreen
+```typescript
+// ❌ WRONG - Violates browser security, causes errors
+useEffect(() => {
+  enterFullscreen() // Browser blocks this without user gesture
+}, [])
+```
+
+**Why this fails:**
+- Browser security requires user gestures for fullscreen
+- Automatic fullscreen attempts cause "Permissions check failed" errors
+- Client-side navigation makes this unnecessary anyway
+
+## Code Review Checklist
+
+Before any PR/commit, verify:
+
+- [ ] All navigation uses `useRouter().push()` instead of `window.location.href`
+- [ ] No automatic fullscreen entry in useEffect hooks
+- [ ] No complex fullscreen restoration prompts (should be unnecessary)
+- [ ] Import `useRouter` from `next/navigation` (not `next/router`)
+
+## Common Anti-Patterns to Avoid
+
+### 1. The "Quick Fix" Trap
+```typescript
+// Don't do this even if "it works"
+const handleClick = () => {
+  window.location.href = '/somewhere' // ❌ Lazy
+}
+
+// Do this instead
+const handleClick = () => {
+  router.push('/somewhere') // ✅ Proper
+}
+```
+
+### 2. The "Copy-Paste" Mistake
+- If you see `window.location.href` in existing code, **fix it**, don't copy it
+- One bad pattern can infect the entire codebase
+
+### 3. The "State Preservation" Hack
+```typescript
+// Don't try to manually preserve state across page reloads
+sessionStorage.setItem('someState', value) // ❌ Unnecessary hack
+
+// Just use proper navigation and state persists automatically
+router.push('/route') // ✅ State preserved
+```
+
+## Performance Benefits
+
+Using proper Next.js navigation provides:
+- **Instant navigation** (no page reload)
+- **Prefetching** of linked routes
+- **Preserved React state** across routes
+- **Better Core Web Vitals** scores
+- **Maintained fullscreen** and other browser states
+
+## Migration Guide
+
+If you find `window.location.href` in the codebase:
+
+1. Add `useRouter` import:
+   ```typescript
+   import { useRouter } from 'next/navigation'
+   ```
+
+2. Get router instance in component:
+   ```typescript
+   const router = useRouter()
+   ```
+
+3. Replace the navigation:
+   ```typescript
+   // Before
+   window.location.href = '/path'
+
+   // After
+   router.push('/path')
+   ```
+
+4. Test that state is preserved across navigation
+
+## Exception Cases
+
+The **ONLY** time you should use `window.location.href` is for:
+- External links (different domains)
+- Full page refreshes (rare, must be justified)
+- Server-side redirects (outside React components)
+
+For everything else: **Use the router!**
+
+---
+
+**Remember:** We chose Next.js for its routing capabilities. Using `window.location.href` is like buying a car and pushing it everywhere instead of using the engine. 🚗➡️🦶
+
+*Last updated: When we fixed the fullscreen persistence issue by actually using Next.js properly*