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/types/index.ts

@@ -41,6 +41,7 @@ export interface NodeData extends Record<string, unknown> {
   notes?: string
   parent_id?: string
   container_mode?: boolean
+  custom_colors?: { border?: string; background?: string; icon?: string }
 }
 
 export interface EdgeData extends Record<string, unknown> {