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/utils/nodeColors.ts

@@ -0,0 +1,31 @@
+import type { NodeData, NodeType } from '@/types'
+
+export interface NodeColors {
+  border: string
+  background: string
+  icon: string
+}
+
+export const NODE_DEFAULT_COLORS: Record<NodeType, NodeColors> = {
+  isp:     { border: '#00d4ff', background: '#21262d', icon: '#00d4ff' },
+  router:  { border: '#00d4ff', background: '#21262d', icon: '#00d4ff' },
+  switch:  { border: '#39d353', background: '#21262d', icon: '#39d353' },
+  server:  { border: '#a855f7', background: '#21262d', icon: '#a855f7' },
+  proxmox: { border: '#ff6e00', background: '#21262d', icon: '#ff6e00' },
+  vm:      { border: '#a855f7', background: '#21262d', icon: '#a855f7' },
+  lxc:     { border: '#00d4ff', background: '#21262d', icon: '#00d4ff' },
+  nas:     { border: '#39d353', background: '#21262d', icon: '#39d353' },
+  iot:     { border: '#e3b341', background: '#21262d', icon: '#e3b341' },
+  ap:      { border: '#00d4ff', background: '#21262d', icon: '#00d4ff' },
+  generic: { border: '#8b949e', background: '#21262d', icon: '#8b949e' },
+}
+
+export function resolveNodeColors(data: Pick<NodeData, 'type' | 'custom_colors'>): NodeColors {
+  const defaults = NODE_DEFAULT_COLORS[data.type] ?? NODE_DEFAULT_COLORS.generic
+  const custom = data.custom_colors
+  return {
+    border:     custom?.border     ?? defaults.border,
+    background: custom?.background ?? defaults.background,
+    icon:       custom?.icon       ?? defaults.icon,
+  }
+}