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/canvasStore.ts

@@ -32,6 +32,7 @@ interface CanvasState {
   editingGroupRectId: string | null
   setEditingGroupRectId: (id: string | null) => void
   markSaved: () => void
+  markUnsaved: () => void
   loadCanvas: (nodes: Node<NodeData>[], edges: Edge<EdgeData>[]) => void
   notifyScanDeviceFound: () => void
 }
@@ -155,6 +156,8 @@ export const useCanvasStore = create<CanvasState>((set) => ({
 
   markSaved: () => set({ hasUnsavedChanges: false }),
 
+  markUnsaved: () => set({ hasUnsavedChanges: true }),
+
   notifyScanDeviceFound: () => set({ scanEventTs: Date.now() }),
 
   loadCanvas: (nodes, edges) => {