fix(desktop): collapse mixed membership arrivals and departures into one timeline row

[pete-lucern]

Problem

The desktop timeline split contiguous membership activity whenever an admin removal or member departure appeared, producing alternating single rows during bursts of channel membership changes.

Change

Contiguous member_joined, member_removed, and member_left events now collapse into one anchored timeline row when they stay within the existing adjacency window and do not cross a barrier. Mixed rows render deduplicated added and removed member name lists with the existing truncation behavior. Pure-arrival and self-join-then-leave summaries retain their existing copy.

Verification

• Alternating 20-event burst: 20 rows before → 1 row after.
• Two membership bursts separated by an ordinary message: 4 rows before → 2 rows after.
• Events more than one hour apart: 2 rows before → 2 rows after.
• Focused touched-file tests: 44 passed, 0 failed.
• Full desktop test script: 6,497 passed, 15 failed; the 15 failures are unrelated existing agent, inbox, thread-reply, and project Git-view tests.
• Biome check passes on all touched files.
• Screenshots not required.

Closest existing PR: none found

[github-actions]

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