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/components/layout/Sidebar.tsx

@@ -17,11 +17,9 @@ import {
   IconChartAreaLine,
   IconClipboardCheck,
   IconSparkles,
-  IconHeartRateMonitor,
   IconCalculator,
   IconGitCompare,
   IconScale,
-  IconSettings,
 } from '@tabler/icons-react';
 import { useAuthStore } from '../../stores/authStore';
 
@@ -47,14 +45,6 @@ const navSections = [
       { label: 'Assessment Groups', icon: IconCategory, path: '/assessment-groups', tourId: 'nav-assessment-groups' },
     ],
   },
-  {
-    label: 'Transactions',
-    items: [
-      { label: 'Transactions', icon: IconReceipt, path: '/transactions', tourId: 'nav-transactions' },
-      { label: 'Invoices', icon: IconFileInvoice, path: '/invoices' },
-      { label: 'Payments', icon: IconCash, path: '/payments' },
-    ],
-  },
   {
     label: 'Board Planning',
     items: [
@@ -68,12 +58,8 @@ const navSections = [
       {
         label: 'Assessment Scenarios', icon: IconCalculator, path: '/board-planning/assessments',
       },
-      {
-        label: 'Investment Planning', icon: IconSparkles, path: '/investment-planning', tourId: 'nav-investment-planning',
-        children: [
-          { label: 'Investment Scenarios', path: '/board-planning/investments' },
-        ],
-      },
+      { label: 'Investment Planning', icon: IconSparkles, path: '/investment-planning', tourId: 'nav-investment-planning' },
+      { label: 'Investment Scenarios', icon: IconScale, path: '/board-planning/investments' },
       { label: 'Compare Scenarios', icon: IconGitCompare, path: '/board-planning/compare' },
     ],
   },
@@ -83,6 +69,14 @@ const navSections = [
       { label: 'Vendors', icon: IconUsers, path: '/vendors' },
     ],
   },
+  {
+    label: 'Transactions',
+    items: [
+      { label: 'Transactions', icon: IconReceipt, path: '/transactions', tourId: 'nav-transactions' },
+      { label: 'Invoices', icon: IconFileInvoice, path: '/invoices' },
+      { label: 'Payments', icon: IconCash, path: '/payments' },
+    ],
+  },
   {
     label: 'Reports',
     items: [
@@ -103,12 +97,6 @@ const navSections = [
       },
     ],
   },
-  {
-    label: 'Account',
-    items: [
-      { label: 'Settings', icon: IconSettings, path: '/settings' },
-    ],
-  },
 ];
 
 interface SidebarProps {