feat(dashboard): add an Unlink action to session cards

[rmyndharis]

Description

Completes the logout work from #984 on the dashboard side: the Sessions page had no way to reach POST /sessions/:id/logout, the only lifecycle operation that tells WhatsApp anything — stop and delete release the session locally but leave the device listed under the account holder's Linked Devices on the phone.

• Button — labelled Unlink (not Logout) so it cannot be confused with the sidebar's own authentication logout, with a matching Unlink lucide icon, styled as a danger action. It renders for exactly the states that have a live engine (ready, initializing, connecting, qr_ready) — the same visibility as Stop, since the API rejects a logout for a session that is not started.
• Confirmation dialog — mirrors the force-kill modal, and spells out both consequences: reconnecting requires a fresh QR scan or pairing code, and local data only goes away with a separate Delete (unlink first, delete after, for a full removal).
• 502 handling — a session that stopped locally but whose unlink WhatsApp did not confirm surfaces as a warning toast with retry guidance (start the session, try again) instead of a plain error, since the end state is not a pure failure.
• Testability — the visibility predicate (canUnlinkSession) and error classification (classifyUnlinkError) live in a pure util (dashboard/src/utils/sessionUnlink.ts) with colocated unit tests, following the dashboard's utils-only test convention.
• i18n — all twelve locales gain sessions.actions.unlink and the sessions.unlink dialog/toast copy.
• Docs — the docs/17 session-card mock-up (which sketched this button as [Logout]) is updated, plus a CHANGELOG entry.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• Tests added/updated
• Documentation updated
• Lint passes
• Self-reviewed

Related Issues

Relates to #984