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

backend/app/schemas/nodes.py

@@ -29,6 +29,7 @@ class NodeBase(BaseModel):
     show_hardware: bool = False
     width: float | None = None
     height: float | None = None
+    bottom_handles: int = 1
 
 
 class NodeCreate(NodeBase):
@@ -60,6 +61,7 @@ class NodeUpdate(BaseModel):
     show_hardware: bool | None = None
     width: float | None = None
     height: float | None = None
+    bottom_handles: int | None = None
 
 
 class NodeResponse(NodeBase):