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/utils/canvasSerializer.ts

@@ -28,6 +28,7 @@ export interface ApiNode extends Record<string, unknown> {
   ram_gb?: number | null
   disk_gb?: number | null
   show_hardware?: boolean
+  properties?: unknown[] | null
   width?: number | null
   height?: number | null
   bottom_handles?: number
@@ -100,6 +101,7 @@ export function serializeNode(n: Node<NodeData>): Record<string, unknown> {
     ram_gb: n.data.ram_gb ?? null,
     disk_gb: n.data.disk_gb ?? null,
     show_hardware: n.data.show_hardware ?? false,
+    properties: n.data.properties ?? [],
     width: n.width ?? null,
     height: n.height ?? null,
     bottom_handles: n.data.bottom_handles ?? 1,