fix(ui): improve elapsed time handling during scans

Signed-off-by: Deluan <deluan@navidrome.org>

ui/src/layout/useScanElapsedTime.test.jsx

@@ -47,4 +47,75 @@ describe('useScanElapsedTime', () => {
 
     expect(result.current).toBe(3e9)
   })
+
+  it('initializes with server value when scan starts', () => {
+    const { result, rerender } = renderHook(
+      ({ scanning, elapsed }) => useScanElapsedTime(scanning, elapsed),
+      {
+        initialProps: { scanning: false, elapsed: 5e9 },
+      },
+    )
+
+    // Start scanning with a new elapsed time from server
+    rerender({ scanning: true, elapsed: 10e9 })
+
+    // Should use the server value when starting
+    expect(result.current).toBe(10e9)
+
+    act(() => {
+      vi.advanceTimersByTime(2000)
+    })
+
+    // Should continue from server value
+    expect(result.current).toBe(12e9)
+  })
+
+  it('ignores server updates during scanning', () => {
+    const { result, rerender } = renderHook(
+      ({ scanning, elapsed }) => useScanElapsedTime(scanning, elapsed),
+      {
+        initialProps: { scanning: true, elapsed: 0 },
+      },
+    )
+
+    act(() => {
+      vi.advanceTimersByTime(3000)
+    })
+
+    expect(result.current).toBe(3e9)
+
+    // Server sends updated elapsed time during scan
+    rerender({ scanning: true, elapsed: 10e9 })
+
+    // Should ignore server update while scanning
+    expect(result.current).toBe(3e9)
+
+    act(() => {
+      vi.advanceTimersByTime(1000)
+    })
+
+    // Should continue from local timer
+    expect(result.current).toBe(4e9)
+  })
+
+  it('uses final server value when scan ends', () => {
+    const { result, rerender } = renderHook(
+      ({ scanning, elapsed }) => useScanElapsedTime(scanning, elapsed),
+      {
+        initialProps: { scanning: true, elapsed: 0 },
+      },
+    )
+
+    act(() => {
+      vi.advanceTimersByTime(3000)
+    })
+
+    expect(result.current).toBe(3e9)
+
+    // Scan ends with final server value
+    rerender({ scanning: false, elapsed: 5e9 })
+
+    // Should use the final server value
+    expect(result.current).toBe(5e9)
+  })
 })