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/__tests__/handleUtils.test.ts

@@ -0,0 +1,111 @@
+import { describe, it, expect } from 'vitest'
+import {
+  BOTTOM_HANDLE_IDS,
+  BOTTOM_HANDLE_POSITIONS,
+  normalizeHandle,
+  removedBottomHandleIds,
+} from '../handleUtils'
+
+describe('BOTTOM_HANDLE_IDS', () => {
+  it('first id is always "bottom" for backward compatibility', () => {
+    expect(BOTTOM_HANDLE_IDS[0]).toBe('bottom')
+  })
+
+  it('has ids for 1–4 handles', () => {
+    expect(BOTTOM_HANDLE_IDS).toHaveLength(4)
+    expect(BOTTOM_HANDLE_IDS).toEqual(['bottom', 'bottom-2', 'bottom-3', 'bottom-4'])
+  })
+})
+
+describe('BOTTOM_HANDLE_POSITIONS', () => {
+  it('1 handle is centered at 50%', () => {
+    expect(BOTTOM_HANDLE_POSITIONS[1]).toEqual([50])
+  })
+
+  it('2 handles are symmetric', () => {
+    const [a, b] = BOTTOM_HANDLE_POSITIONS[2]
+    expect(a).toBeLessThan(50)
+    expect(b).toBeGreaterThan(50)
+    expect(a + b).toBe(100)
+  })
+
+  it('3 handles include a center at 50%', () => {
+    expect(BOTTOM_HANDLE_POSITIONS[3]).toContain(50)
+    expect(BOTTOM_HANDLE_POSITIONS[3]).toHaveLength(3)
+  })
+
+  it('4 handles are evenly spaced', () => {
+    const pos = BOTTOM_HANDLE_POSITIONS[4]
+    expect(pos).toHaveLength(4)
+    // All values should be between 0 and 100 exclusive
+    pos.forEach((p) => {
+      expect(p).toBeGreaterThan(0)
+      expect(p).toBeLessThan(100)
+    })
+    // Positions should be strictly increasing
+    for (let i = 1; i < pos.length; i++) {
+      expect(pos[i]).toBeGreaterThan(pos[i - 1])
+    }
+  })
+})
+
+describe('normalizeHandle', () => {
+  it('returns null for null/undefined', () => {
+    expect(normalizeHandle(null)).toBeNull()
+    expect(normalizeHandle(undefined)).toBeNull()
+  })
+
+  it('maps top-t → top', () => {
+    expect(normalizeHandle('top-t')).toBe('top')
+  })
+
+  it('maps bottom-t → bottom', () => {
+    expect(normalizeHandle('bottom-t')).toBe('bottom')
+  })
+
+  it('maps bottom-2-t → bottom-2', () => {
+    expect(normalizeHandle('bottom-2-t')).toBe('bottom-2')
+  })
+
+  it('maps bottom-3-t → bottom-3', () => {
+    expect(normalizeHandle('bottom-3-t')).toBe('bottom-3')
+  })
+
+  it('maps bottom-4-t → bottom-4', () => {
+    expect(normalizeHandle('bottom-4-t')).toBe('bottom-4')
+  })
+
+  it('passes through non-stub handles unchanged', () => {
+    expect(normalizeHandle('top')).toBe('top')
+    expect(normalizeHandle('bottom')).toBe('bottom')
+    expect(normalizeHandle('bottom-2')).toBe('bottom-2')
+    expect(normalizeHandle('custom-handle')).toBe('custom-handle')
+  })
+})
+
+describe('removedBottomHandleIds', () => {
+  it('returns empty set when count does not decrease', () => {
+    expect(removedBottomHandleIds(2, 2).size).toBe(0)
+    expect(removedBottomHandleIds(1, 4).size).toBe(0)
+  })
+
+  it('4 → 1 removes bottom-2, bottom-3, bottom-4', () => {
+    const removed = removedBottomHandleIds(4, 1)
+    expect(removed).toEqual(new Set(['bottom-2', 'bottom-3', 'bottom-4']))
+  })
+
+  it('4 → 2 removes bottom-3, bottom-4', () => {
+    const removed = removedBottomHandleIds(4, 2)
+    expect(removed).toEqual(new Set(['bottom-3', 'bottom-4']))
+  })
+
+  it('3 → 2 removes only bottom-3', () => {
+    const removed = removedBottomHandleIds(3, 2)
+    expect(removed).toEqual(new Set(['bottom-3']))
+  })
+
+  it('never removes "bottom" (index 0)', () => {
+    const removed = removedBottomHandleIds(4, 1)
+    expect(removed.has('bottom')).toBe(false)
+  })
+})