Restore neutral colors for default palette

Remove indigo tints from default palette light/dark themes.
Use neutral slate-gray colors for backgrounds and text, letting
the accent color provide visual highlights in interactive elements.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/contexts/ThemeContext.tsx

@@ -193,24 +193,24 @@ export type CustomColors = Partial<{
 export const COLOR_PALETTES: Record<ColorPalette, PaletteColors> = {
   default: {
     light: {
-      bgMain: '#f8f9ff',
+      bgMain: '#f7fafc',
       bgCard: '#ffffff',
-      bgElevated: '#fafbff',
-      textPrimary: '#1a1f3a',
-      textSecondary: '#4a5578',
-      border: '#e5e7f4',
-      sidebarBg: '#3949ab',
+      bgElevated: '#f8fafc',
+      textPrimary: '#1a202c',
+      textSecondary: '#4a5568',
+      border: '#e2e8f0',
+      sidebarBg: '#2d3748',
       sidebarText: '#f9fafb',
     },
     dark: {
-      bgMain: '#0f1229',
-      bgCard: '#1a1f3f',
-      bgElevated: '#252b4d',
-      textPrimary: '#e8eaf6',
-      textSecondary: '#9fa8da',
-      border: '#303652',
-      sidebarBg: '#0a0d1f',
-      sidebarText: '#e8eaf6',
+      bgMain: '#0f172a',
+      bgCard: '#1e293b',
+      bgElevated: '#334155',
+      textPrimary: '#f1f5f9',
+      textSecondary: '#cbd5e1',
+      border: '#334155',
+      sidebarBg: '#0c1222',
+      sidebarText: '#f1f5f9',
     },
   },
   monochrome: {