feat: replace static hardware fields with dynamic node properties

Replaces the 4 fixed hardware columns (cpu_count, cpu_model, ram_gb,
disk_gb) with a flexible properties system. Each property has a key,
value, icon (from a curated Lucide picker), and a visibility toggle
that controls whether it appears on the canvas node card.

- Backend: add `properties` JSON column to Node model; data migration
  converts existing hardware rows to properties with correct icons
  (idempotent, old columns kept for safety)
- Backend: add `properties` to NodeBase, NodeUpdate, NodeSave schemas
  and canvasSerializer so values survive canvas save/load
- Frontend: add NodeProperty type; new propertyIcons.ts registry (20
  icons); BaseNode renders visible properties with legacy hardware
  fallback for unmigrated nodes
- Frontend: DetailPanel gains interactive properties section (add /
  edit / remove / toggle visibility / icon picker) replacing the
  read-only hardware block; hardware section removed from NodeModal
- Tests: 6 migration tests, 7 API tests, 8 DetailPanel property tests,
  6 BaseNode render/fallback tests, 9 propertyIcons util tests

frontend/src/types/index.ts

@@ -43,6 +43,13 @@ export interface ServiceInfo {
   category?: string
 }
 
+export interface NodeProperty {
+  key: string
+  value: string
+  icon: string | null
+  visible: boolean
+}
+
 export interface NodeData extends Record<string, unknown> {
   label: string
   type: NodeType
@@ -62,6 +69,7 @@ export interface NodeData extends Record<string, unknown> {
   ram_gb?: number
   disk_gb?: number
   show_hardware?: boolean
+  properties?: NodeProperty[]
   parent_id?: string
   container_mode?: boolean
   custom_colors?: {