feat: add custom icon picker to node create/edit modal

- Add 65+ icons across 7 categories (Infrastructure, Media, Monitoring,
  Storage, Security, Automation, Dev & Containers, Communications) covering
  popular self-hosted apps: Home Assistant, Jellyfin, Plex, Grafana, Portainer,
  Pi-hole, Vaultwarden, Gitea, Nextcloud, Node-RED, Frigate, etc.
- New nodeIcons.ts utility with ICON_REGISTRY, ICON_MAP and resolveNodeIcon()
- Inline icon picker in NodeModal: collapsible panel with search + grid grouped
  by category; click to select, click again or Reset to revert to type default
- BaseNode uses resolveNodeIcon() so custom icon renders live on canvas
- Add custom_icon field to NodeData type, NodeBase/NodeUpdate schemas, Node ORM
  model, and database.py idempotent ALTER TABLE migration

backend/app/db/database.py

@@ -30,6 +30,8 @@ async def init_db() -> None:
             await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN custom_color TEXT")
         with suppress(Exception):
             await conn.exec_driver_sql("ALTER TABLE edges ADD COLUMN path_style TEXT")
+        with suppress(Exception):
+            await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN custom_icon TEXT")
 
 
 async def get_db() -> AsyncGenerator[AsyncSession, None]: