fix(desktop): collapse consecutive member removals into one system row

[tadeo-ctrl]

Problem

Desktop already collapses a run of contiguous member_joined events into one summary row — buildMembershipGroups in timelineItems.ts plus the members_added / members_joined branches in SystemMessageRow.tsx. The matching member_removed case was never wired up, so each removal takes its own timeline row.

That reads fine for the one-off removal the copy was written for. It reads badly for the bulk case, which is ordinary — offboarding a team, or clearing the placeholder identities left behind by a workspace import. Removing eleven members leaves eleven near-identical rows pinned to the bottom of the channel.

There is also no way to clean them up after the fact: deleting a system row publishes a message_deleted system row in its place, so the row count never drops.

Change

Group contiguous member_removed events performed by the same administrator, reusing the machinery that already backs members_added — the same one-hour adjacency window, the same day/unread barriers, the same avatar stack and "and N others" tooltip.

• timelineItems.ts — parseMembershipChangePayload recognises member_removed as a new removal mode; membershipChangesCanGroup joins removals only to removals by the same actor.
• SystemMessageRow.tsx — buildGroupedRemovalPayload emits a members_removed payload, with a describeSystemEvent branch rendering it through the existing MemberNamesInlineList.

Removals do not group with additions, and two different moderators still render separately, so no existing sequence changes shape.

Tests

• timelineItems.test.mjs — three cases: contiguous removals by one actor group; removals by different actors do not; removals do not group with additions.
• mentions.spec.ts — groups contiguous removal activity with hidden names in the standard tooltip, mirroring the existing arrival-grouping spec, covering the collapsed copy, the avatar stack and the overflow tooltip.

Gates run locally: pnpm test (4390 passing), pnpm typecheck, pnpm check, and the full smoke run of mentions.spec.ts (59 passing).

Screenshots in a comment below.

[tadeo-ctrl]

Before — six removals by one admin, six rows

After — one row

Captured from the mentions.spec.ts removal-grouping spec added in this PR, run against main (before) and this branch (after).

Note: scripts/post-screenshots.sh pushes to origin, which an outside fork contributor cannot write to, so these blobs live on the same immutable commit-SHA scheme on the fork instead.