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/main.py

@@ -1,3 +1,5 @@
+import logging
+import logging.config
 from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager
 from typing import Any
@@ -14,6 +16,16 @@ from app.db.database import init_db
 
 @asynccontextmanager
 async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
+    # Ensure app logs are visible: attach a handler to the root logger if none
+    # exists (uvicorn only installs handlers on its own loggers, not the root).
+    root_logger = logging.getLogger()
+    if not root_logger.handlers:
+        handler = logging.StreamHandler()
+        handler.setFormatter(logging.Formatter("%(levelname)s:%(name)s:%(message)s"))
+        root_logger.addHandler(handler)
+    root_logger.setLevel(logging.INFO)
+    logging.getLogger("app").setLevel(logging.INFO)
+    logging.getLogger("app.services.scanner").setLevel(logging.INFO)
     await init_db()
     settings.load_overrides()
     start_scheduler()