fix: add Expense doctype link to Expenses workspace sidebar

Problem

The Expense doctype had no entry point in the Expenses workspace sidebar on v5, even though Expense Claim already had one.

Root cause

The v5 Expenses workspace itself already links to Expense via the "Claims" card, but the corresponding workspace sidebar was never updated with the same link.

Fix

Add an Expense link to the Expenses workspace sidebar, next to Expense Claim. This backports the sidebar half of the equivalent fix on develop (MR !2624 (merged)), adapted to v5's legacy sidebar schema.