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/stores/preferencesStore.ts

@@ -5,19 +5,26 @@ type ColorScheme = 'light' | 'dark';
 
 interface PreferencesState {
   colorScheme: ColorScheme;
+  compactView: boolean;
   toggleColorScheme: () => void;
   setColorScheme: (scheme: ColorScheme) => void;
+  toggleCompactView: () => void;
+  setCompactView: (compact: boolean) => void;
 }
 
 export const usePreferencesStore = create<PreferencesState>()(
   persist(
     (set) => ({
       colorScheme: 'light',
+      compactView: false,
       toggleColorScheme: () =>
         set((state) => ({
           colorScheme: state.colorScheme === 'light' ? 'dark' : 'light',
         })),
       setColorScheme: (scheme) => set({ colorScheme: scheme }),
+      toggleCompactView: () =>
+        set((state) => ({ compactView: !state.compactView })),
+      setCompactView: (compact) => set({ compactView: compact }),
     }),
     {
       name: 'ledgeriq-preferences',