Add BOM cost tracking

Suggested-default cost on CableType/DeviceTemplate, copied onto
placed Device instances (same pattern as ports) and overridable
per-connection/per-device in the diagram. BOM view shows per-line
and grand-total cost, excluding (and flagging) anything without a
computable cost rather than treating it as zero.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017DUU6CnxECCDeqDNYJgr5x

src/components/inspector/DeviceInspector.tsx

@@ -49,7 +49,7 @@ export default function DeviceInspector({ deviceId }: { deviceId: string }) {
             className="mt-1 w-full rounded border border-slate-300 px-2 py-1 text-sm"
           />
         </label>
-        <label className="col-span-2 text-xs font-medium text-slate-600">
+        <label className="text-xs font-medium text-slate-600">
           Model
           <input
             value={device.model ?? ''}
@@ -57,6 +57,23 @@ export default function DeviceInspector({ deviceId }: { deviceId: string }) {
             className="mt-1 w-full rounded border border-slate-300 px-2 py-1 text-sm"
           />
         </label>
+        <label className="text-xs font-medium text-slate-600">
+          Cost
+          <div className="mt-1 flex items-center gap-1">
+            <span className="text-xs text-slate-500">$</span>
+            <input
+              type="number"
+              min={0}
+              step={0.01}
+              value={device.cost ?? ''}
+              onChange={(e) =>
+                updateDevice(device.id, { cost: e.target.value === '' ? undefined : Number(e.target.value) })
+              }
+              className="w-full rounded border border-slate-300 px-2 py-1 text-sm"
+              placeholder="e.g. 249.00"
+            />
+          </div>
+        </label>
         <label className="col-span-2 text-xs font-medium text-slate-600">
           Notes
           <textarea