Full health check: fix startup crash, add subnet input to network discovery

Server fixes:
- database.js: auto-create data/ directory on startup — was crashing on
  fresh clone because the gitignored directory didn't exist
- better-sqlite3 upgraded to latest (13.x) for Node 26 compatibility

Network discovery:
- discoverNetwork() accepts optional subnet param; scans that range instead
  of auto-detecting when provided
- New GET /api/network-subnets endpoint returns detected local interfaces
  so UI can offer quick-select buttons
- Discovery modal: subnet text input + Scan button + auto-detect quick-select;
  local subnet buttons populate once the modal is open
- server/index.js: pass subnet from request body to discoverNetwork()

Infra:
- React client runs on PORT=3456 in dev to avoid port conflicts
- .claude/launch.json added for one-command preview via Express (serves
  the pre-built client, no port conflict with the preview tool)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/networkDiscovery.js

@@ -101,20 +101,29 @@ async function getArpHosts() {
   });
 }
 
-async function discoverNetwork() {
-  const subnets = getLocalSubnets();
+async function discoverNetwork(customSubnet = null) {
   const arpHosts = await getArpHosts();
   const arpIps = new Set(arpHosts.map(h => h.ip));
 
-  // Collect all IPs to scan: ARP table + subnet range
   const allIps = new Map();
-  for (const h of arpHosts) allIps.set(h.ip, { ip: h.ip, hostname: h.hostname });
 
-  for (const subnet of subnets) {
-    if (!subnet.prefix) continue;
-    const range = subnetToRange(subnet.prefix);
-    for (const ip of range) {
-      if (!allIps.has(ip)) allIps.set(ip, { ip, hostname: null });
+  if (customSubnet) {
+    // Scan the user-specified subnet only (no ARP seeding, still probe all IPs in range)
+    const range = subnetToRange(customSubnet);
+    for (const ip of range) allIps.set(ip, { ip, hostname: null });
+    // Supplement with ARP hostnames for any IPs that happen to be in the ARP table
+    for (const h of arpHosts) {
+      if (allIps.has(h.ip) && h.hostname) allIps.get(h.ip).hostname = h.hostname;
+    }
+  } else {
+    // Auto-detect: ARP table + all local subnets
+    for (const h of arpHosts) allIps.set(h.ip, { ip: h.ip, hostname: h.hostname });
+    for (const subnet of getLocalSubnets()) {
+      if (!subnet.prefix) continue;
+      const range = subnetToRange(subnet.prefix);
+      for (const ip of range) {
+        if (!allIps.has(ip)) allIps.set(ip, { ip, hostname: null });
+      }
     }
   }