feat: configurable bottom handles, scanner rewrite, UI polish

## New features
- Configurable bottom connection points per node (1–4 handles)
- Fit view on load
- LiveView improvements
- Node modal: inline Type/Icon picker, default icon in trigger
- Remove redundant Save button from ScanConfigModal

## Scanner fixes
- Phase 1: replace nmap ARP sweep with concurrent asyncio ping sweep
  (50 parallel pings, 1s timeout). Zero false positives, works in any
  Docker network mode. Supplements with /proc/net/arp for ICMP-blocked devices.
- Phase 2: explicit -sS (root) / -sT (non-root) scan type; bump
  host-timeout to 60s; gather(return_exceptions=True) so one failing
  host doesn't abort the batch
- Fix 404 on missing device in hide/ignore
- Validate CIDR ranges to prevent nmap injection
- Thread-safe cancel set, pre-fetch canvas/hidden IPs (no N+1 queries)
- Logging: attach StreamHandler to root logger so app.* logs are visible

## Tests
- 21 backend scanner tests (ping sweep, ARP cache, Phase 2 tolerance)
- Full NodeModal coverage (53 tests)
- LiveView, store, edge label tests

frontend/src/utils/nodeIcons.ts

@@ -1,3 +1,4 @@
+import type { NodeType } from '@/types'
 import {
   // Infrastructure (node types)
   Globe, Router, Network, Server, Layers, Box, Container, HardDrive, Cpu, Wifi, Circle,
@@ -116,6 +117,27 @@ export const ICON_MAP: Record<string, LucideIcon> = Object.fromEntries(
   ICON_REGISTRY.map((e) => [e.key, e.icon]),
 )
 
+export const NODE_TYPE_DEFAULT_ICONS: Record<NodeType, LucideIcon> = {
+  isp:      Globe,
+  router:   Router,
+  switch:   Network,
+  server:   Server,
+  proxmox:  Layers,
+  vm:       Box,
+  lxc:      Container,
+  nas:      HardDrive,
+  iot:      Cpu,
+  ap:       Wifi,
+  camera:   Cctv,
+  printer:  Printer,
+  computer: Monitor,
+  cpl:      PlugZap,
+  docker:   Anchor,
+  generic:  Circle,
+  group:    Circle,
+  groupRect: Circle,
+}
+
 /** Resolve the display icon for a node — custom_icon takes priority over type default. */
 export function resolveNodeIcon(
   typeIcon: LucideIcon,