feat: add logo, favicon, and theme system

- Add custom SVG favicon and Logo component (house + network nodes motif)
- Update page title to Homelable with meta description
- Show Logo in sidebar header and toolbar
- Add theme store and ThemeModal for canvas style switching
- Refactor node colors and edge styles for theme support

frontend/src/stores/__tests__/themeStore.test.ts

@@ -0,0 +1,31 @@
+import { describe, it, expect, beforeEach } from 'vitest'
+import { useThemeStore } from '@/stores/themeStore'
+
+describe('themeStore', () => {
+  beforeEach(() => {
+    useThemeStore.setState({ activeTheme: 'default' })
+  })
+
+  it('starts with default theme', () => {
+    expect(useThemeStore.getState().activeTheme).toBe('default')
+  })
+
+  it('setTheme updates activeTheme', () => {
+    useThemeStore.getState().setTheme('matrix')
+    expect(useThemeStore.getState().activeTheme).toBe('matrix')
+  })
+
+  it('setTheme can switch between all presets', () => {
+    const themes = ['default', 'dark', 'light', 'neon', 'matrix'] as const
+    for (const id of themes) {
+      useThemeStore.getState().setTheme(id)
+      expect(useThemeStore.getState().activeTheme).toBe(id)
+    }
+  })
+
+  it('setTheme back to default after neon', () => {
+    useThemeStore.getState().setTheme('neon')
+    useThemeStore.getState().setTheme('default')
+    expect(useThemeStore.getState().activeTheme).toBe('default')
+  })
+})