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/components/canvas/edges/index.tsx

@@ -26,7 +26,7 @@ export function HomelableEdge({ id, source, target, sourceX, sourceY, targetX, t
   const isBidirectional = sourceType === 'proxmox' && targetType === 'proxmox'
 
   const pathArgs = { sourceX, sourceY, sourcePosition, targetX, targetY, targetPosition }
-  const [edgePath, labelX, labelY] = data?.path_style === 'smooth'
+  const [edgePath, labelX] = data?.path_style === 'smooth'
     ? getSmoothStepPath({ ...pathArgs, borderRadius: 8 })
     : getBezierPath(pathArgs)
 
@@ -95,9 +95,9 @@ export function HomelableEdge({ id, source, target, sourceX, sourceY, targetX, t
       {data?.label && (
         <EdgeLabelRenderer>
           <div
-            className="absolute pointer-events-none font-mono text-[10px] px-1 rounded"
+            className="absolute pointer-events-none font-mono text-[10px] px-1.5 py-0.5 rounded"
             style={{
-              transform: `translate(-50%, -50%) translate(${labelX}px, ${labelY}px)`,
+              transform: `translate(-50%, -50%) translate(${labelX}px, ${(sourceY + targetY) / 2}px)`,
               background: theme.colors.edgeLabelBackground,
               color:      theme.colors.edgeLabelColor,
               border:     `1px solid ${theme.colors.edgeLabelBorder}`,