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/pages/preferences/UserPreferencesPage.tsx

@@ -10,7 +10,7 @@ import { usePreferencesStore } from '../../stores/preferencesStore';
 
 export function UserPreferencesPage() {
   const { user, currentOrg } = useAuthStore();
-  const { colorScheme, toggleColorScheme } = usePreferencesStore();
+  const { colorScheme, toggleColorScheme, compactView, toggleCompactView } = usePreferencesStore();
 
   return (
     <Stack>
@@ -78,7 +78,7 @@ export function UserPreferencesPage() {
                 <Text size="sm">Compact View</Text>
                 <Text size="xs" c="dimmed">Reduce spacing in tables and lists</Text>
               </div>
-              <Switch disabled />
+              <Switch checked={compactView} onChange={toggleCompactView} />
             </Group>
             <Divider />
             <Text size="xs" c="dimmed" ta="center">More display preferences coming in a future release</Text>