feat: per-node custom color styling (border, background, icon)

- Add resolveNodeColors() utility merging type defaults with per-node overrides
- Default colors per node type (cyan=isp/router/lxc/ap, green=switch/nas,
  purple=server/vm, orange=proxmox, amber=iot, gray=generic)
- Remove glowColor prop from BaseNode — colors now come from node data
- ProxmoxGroupNode uses resolveNodeColors for group border/header/icon
- NodeModal: Appearance section with 3 color swatches (border, background, icon)
  — click swatch to open native color picker; Reset to defaults button
- custom_colors persisted as JSON in DB (backend model + schemas + migration)
- 7 new unit tests for resolveNodeColors covering all node types + partial overrides

frontend/src/App.tsx

@@ -52,6 +52,7 @@ export default function App() {
         notes: n.data.notes ?? null,
         parent_id: n.data.parent_id ?? null,
         container_mode: n.data.container_mode ?? false,
+        custom_colors: n.data.custom_colors ?? null,
         pos_x: n.position.x,
         pos_y: n.position.y,
       }))