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/canvasSerializer.ts

@@ -1,5 +1,6 @@
 import type { Node, Edge } from '@xyflow/react'
 import type { NodeData, EdgeData } from '@/types'
+import { normalizeHandle } from '@/utils/handleUtils'
 
 // ── Types ────────────────────────────────────────────────────────────────────
 
@@ -29,6 +30,7 @@ export interface ApiNode extends Record<string, unknown> {
   show_hardware?: boolean
   width?: number | null
   height?: number | null
+  bottom_handles?: number
 }
 
 export interface ApiEdge {
@@ -99,14 +101,12 @@ export function serializeNode(n: Node<NodeData>): Record<string, unknown> {
     show_hardware: n.data.show_hardware ?? false,
     width: n.width ?? null,
     height: n.height ?? null,
+    bottom_handles: n.data.bottom_handles ?? 1,
     pos_x: n.position.x,
     pos_y: n.position.y,
   }
 }
 
-const normalizeHandle = (h: string | null | undefined): string | null =>
-  h === 'top-t' ? 'top' : h === 'bottom-t' ? 'bottom' : (h ?? null)
-
 export function serializeEdge(e: Edge<EdgeData>): Record<string, unknown> {
   return {
     id: e.id,