refactor: remove all backtest module code and references

Delete backtest/ engine (19 files), api/backtest.go, store/backtest.go,
web backtest components (7 files), API client, types, docs, screenshot.
Clean all backtest references from main.go, api/server.go, store/store.go,
App.tsx, HeaderBar.tsx, LandingPage.tsx, translations, README and docs.

web/src/pages/LandingPage.tsx

@@ -41,7 +41,6 @@ export function LandingPage() {
             'strategy-market': '/strategy-market',
             'traders': '/traders',
             'trader': '/dashboard',
-            'backtest': '/backtest',
             'strategy': '/strategy',
             'faq': '/faq',
           }