Add Expense to the client list

[assaf]

Adds Expense to the Clients list on https://jmap.io/software/.

Expense (https://expense.labnotes.org) is a free web expense tracker for
freelancers and the self-employed. It connects to a JMAP mailbox — Fastmail, or
any JMAP server you point it at, discovered over /.well-known/jmap — and turns
the receipts in it into categorized, tax-ready expenses, moving processed mail
to Trash. It uses JMAP Core and Mail: session discovery, Email/query,
Email/get, Email/set, Email/import, blob upload/download, and
PushSubscription for push, authenticated with a Bearer token or HTTP Basic.

The entry is in alphabetical position in the Clients section and is formatted
with npm run lint-prettier.

Only the site's list is touched. I left jmapio/jmap's
software/software.mdown alone: it looks older than this page and is missing
entries that are on the site (Sterna, ihasmail, Pimsync).

[assaf]

Local verification, since the Lint workflow is sitting at action_required (first-time contributor to this repo, so it needs a maintainer to approve the run):

• npm ci && npx prettier --check pages/software.liquid — clean; the full npm run lint-prettier (prettier --check .) also passes, so the change is prettier-formatted with the repo's own config.
• Rendered the site the way _bin/local-build does (Docker ruby:3.3, bundle exec jekyll serve). GET /software/ produces one new card inside the Clients section, between Cypht and Group-Office: eyebrow TypeScript, title Expense, link to https://expense.labnotes.org. Diffing the rendered HTML against a render of master shows the added <li> and nothing else apart from the ?ts= asset cache-busters.
• Front matter parses with the repo's gray-matter: the item lands at index 6 of the Clients items list.

So a Lint run should pass once approved. The Deploy Preview check will not run from this fork.