Fix dynamic colspan in UserDeviceList.vue table layouts

[coderabbitai]

Description

The empty-state table row in UserDeviceList.vue uses a fixed colspan="5" which doesn't match the actual number of columns in either mobile or desktop views.

Current Behavior

• Desktop view renders a table with 3 columns (device, added, lastAccess)
• Mobile view renders a table with 1 column (composite cell with nested mini-table)
• The empty state row (<tr v-if="!filteredDevices.length">) uses colspan="5" which is incorrect for both views

Expected Behavior

The colspan should be dynamic based on the current viewport/rendering mode, or the table structure should be refactored to handle responsive layouts properly.

Context

This issue was identified during code review of PR #376 and discussed in:

• PR: Add user and group management UI #376
• Comment: Add user and group management UI #376 (comment)

Requested by: @overheadhunter

Location

File: frontend/src/components/authority/UserDeviceList.vue
Lines: ~133-137