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/themeStore.ts

@@ -0,0 +1,12 @@
+import { create } from 'zustand'
+import type { ThemeId } from '@/utils/themes'
+
+interface ThemeState {
+  activeTheme: ThemeId
+  setTheme: (id: ThemeId) => void
+}
+
+export const useThemeStore = create<ThemeState>((set) => ({
+  activeTheme: 'default',
+  setTheme: (id) => set({ activeTheme: id }),
+}))