feat: sidebar reorg, compact view preference, and UI polish

- Remove redundant Settings link from sidebar (accessible via user menu)
- Move Transactions section below Board Reference for better grouping
- Promote Investment Scenarios to its own top-level sidebar item
- Add Compact View preference with tighter spacing theme
- Wire compact theme into MantineProvider with dynamic switching
- Enable Compact View toggle in both Preferences and Settings pages
- Install missing @simplewebauthn/browser package (lock file update)

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

frontend/src/main.tsx

@@ -9,7 +9,7 @@ import '@mantine/core/styles.css';
 import '@mantine/dates/styles.css';
 import '@mantine/notifications/styles.css';
 import { App } from './App';
-import { theme } from './theme/theme';
+import { defaultTheme, compactTheme } from './theme/theme';
 import { usePreferencesStore } from './stores/preferencesStore';
 
 const queryClient = new QueryClient({
@@ -24,9 +24,11 @@ const queryClient = new QueryClient({
 
 function Root() {
   const colorScheme = usePreferencesStore((s) => s.colorScheme);
+  const compactView = usePreferencesStore((s) => s.compactView);
+  const activeTheme = compactView ? compactTheme : defaultTheme;
 
   return (
-    <MantineProvider theme={theme} forceColorScheme={colorScheme}>
+    <MantineProvider theme={activeTheme} forceColorScheme={colorScheme}>
       <Notifications position="top-right" />
       <ModalsProvider>
         <QueryClientProvider client={queryClient}>