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/ConnectionInspector.tsx

@@ -20,6 +20,8 @@ export default function ConnectionInspector({ connectionId }: { connectionId: st
     return sourcePortType && targetPortType ? cableTypesForConnection(project, sourcePortType, targetPortType) : []
   }, [project, sourcePort, targetPort])
 
+  const cableCostPerUnit = availableCableTypes.find((ct) => ct.id === connection?.cableTypeId)?.costPerUnit
+
   if (!connection || !sourceDevice || !targetDevice || !sourcePort || !targetPort) return null
 
   return (
@@ -76,6 +78,33 @@ export default function ConnectionInspector({ connectionId }: { connectionId: st
         </div>
       </label>
 
+      <label className="block text-xs font-medium text-slate-600">
+        Cost override
+        <div className="mt-1 flex items-center gap-2">
+          <span className="text-xs text-slate-500">$</span>
+          <input
+            type="number"
+            min={0}
+            step={0.01}
+            value={connection.cost ?? ''}
+            onChange={(e) =>
+              updateConnection(connection.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={
+              cableCostPerUnit !== undefined && connection.distance
+                ? `computed: $${(cableCostPerUnit * connection.distance).toFixed(2)}`
+                : 'e.g. 24.99'
+            }
+          />
+        </div>
+        <p className="mt-1 text-[10px] text-slate-400">
+          Overrides the cable type's per-unit cost for this run specifically. Leave blank to use cost/unit × distance.
+        </p>
+      </label>
+
       <label className="block text-xs font-medium text-slate-600">
         Label
         <input