fix(rack): keep Import links usable after a run that matched nothing

importCablesFromNetwork latched networkImportDone whatever the outcome. Click
Import links before racking anything and the toast says "No matching link found
— rack the devices first", then the button is disabled for the rest of the
session: it asks for a retry and forbids it. Only a design switch or a reload
cleared the flag.

It now latches on success only.

ha-relevant: no

frontend/src/rack/__tests__/store.test.ts

@@ -423,6 +423,19 @@ describe('cables', () => {
     expect(store().importCablesFromNetwork(demoNetworkLinks())).toBe(0)
   })
 
+  it('stays available after an import that matched nothing', () => {
+    // Nothing racked for these hints yet: the run creates nothing, and the user
+    // is told to rack the devices first — so the button has to survive it.
+    expect(store().importCablesFromNetwork([
+      { from: 'node-nowhere-a', to: 'node-nowhere-b', type: 'ethernet' },
+    ])).toBe(0)
+    expect(store().networkImportDone).toBe(false)
+
+    store().mountFromInventory('inv-sw8', 'rack-main', { uStart: 4 })
+    expect(store().importCablesFromNetwork(demoNetworkLinks())).toBeGreaterThan(0)
+    expect(store().networkImportDone).toBe(true)
+  })
+
   it('skips hints whose devices are not racked', () => {
     // inv-jbod is never mounted in the demo, so its hint cannot resolve.
     store().importCablesFromNetwork(demoNetworkLinks())

frontend/src/rack/store.ts

@@ -673,7 +673,10 @@ export const useRackStore = create<RackState>((set, get) => {
         )
         if (id) created++
       }
-      set({ networkImportDone: true })
+      // Only latch on success. An import run before anything is racked matches
+      // nothing and says so — disabling the button then would kill the retry the
+      // toast just asked for.
+      if (created > 0) set({ networkImportDone: true })
       return created
     },