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/components/panels/Toolbar.tsx

@@ -1,22 +1,28 @@
-import { Save, LayoutDashboard, Download } from 'lucide-react'
+import { Save, LayoutDashboard, Download, Palette } from 'lucide-react'
 import { Button } from '@/components/ui/button'
+import { Logo } from '@/components/ui/Logo'
 import { useCanvasStore } from '@/stores/canvasStore'
 
 interface ToolbarProps {
   onSave: () => void
   onAutoLayout: () => void
   onExport: () => void
+  onChangeStyle: () => void
 }
 
-export function Toolbar({ onSave, onAutoLayout, onExport }: ToolbarProps) {
+export function Toolbar({ onSave, onAutoLayout, onExport, onChangeStyle }: ToolbarProps) {
   const { hasUnsavedChanges } = useCanvasStore()
 
   return (
     <header className="flex items-center gap-2 px-4 py-2 border-b border-border bg-[#161b22] shrink-0">
+      <Logo size={28} showText={true} />
       <div className="flex-1" />
       <Button size="sm" variant="ghost" className="gap-1.5 text-muted-foreground hover:text-foreground" onClick={onAutoLayout}>
         <LayoutDashboard size={14} /> Auto Layout
       </Button>
+      <Button size="sm" variant="ghost" className="gap-1.5 text-muted-foreground hover:text-foreground" onClick={onChangeStyle}>
+        <Palette size={14} /> Style
+      </Button>
       <Button size="sm" variant="ghost" className="gap-1.5 text-muted-foreground hover:text-foreground" onClick={onExport}>
         <Download size={14} /> Export
       </Button>