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/theme/theme.ts

@@ -1,10 +1,57 @@
 import { createTheme } from '@mantine/core';
 
-export const theme = createTheme({
+const baseFontFamily = '-apple-system, BlinkMacSystemFont, Segoe UI, Roboto, sans-serif';
+
+export const defaultTheme = createTheme({
   primaryColor: 'blue',
-  fontFamily: '-apple-system, BlinkMacSystemFont, Segoe UI, Roboto, sans-serif',
+  fontFamily: baseFontFamily,
   headings: {
-    fontFamily: '-apple-system, BlinkMacSystemFont, Segoe UI, Roboto, sans-serif',
+    fontFamily: baseFontFamily,
   },
   defaultRadius: 'md',
 });
+
+export const compactTheme = createTheme({
+  primaryColor: 'blue',
+  fontFamily: baseFontFamily,
+  headings: {
+    fontFamily: baseFontFamily,
+  },
+  defaultRadius: 'md',
+  spacing: {
+    xs: '4px',
+    sm: '6px',
+    md: '10px',
+    lg: '12px',
+    xl: '16px',
+  },
+  fontSizes: {
+    xs: '11px',
+    sm: '12px',
+    md: '13px',
+    lg: '15px',
+    xl: '18px',
+  },
+  components: {
+    Table: {
+      defaultProps: {
+        verticalSpacing: 'xs',
+        horizontalSpacing: 'xs',
+        fz: 'sm',
+      },
+    },
+    Card: {
+      defaultProps: {
+        padding: 'sm',
+      },
+    },
+    AppShell: {
+      defaultProps: {
+        padding: 'xs',
+      },
+    },
+  },
+});
+
+/** @deprecated Use `defaultTheme` or `compactTheme` instead */
+export const theme = defaultTheme;