feat(desktop): show activity timestamps on demand

[tellaho]

Category: improvement
User Impact: Users can turn on timestamps in the right-pane Activity feed to see when each agent activity row happened.
Problem: The refreshed Activity feed made rows easier to scan, but it removed the always-visible timing cue people use to tell whether an agent is actively working or stale. The pane needed a way to restore that context without adding visual noise for everyone by default.
Solution: This adds a device-level Show Timestamps setting to the Activity pane header. When enabled, polished activity rows render the existing transcript timestamp style at the bottom-left, and raw activity cards show the same timestamp treatment as a low-emphasis second row inside each card. Compact profile previews stay dense, and user message bubbles keep their existing single timestamp.

File changes

desktop/src/features/agents/ui/AgentSessionTranscriptList.tsx
Adds the optional timestamp footer beneath polished activity rows and grouped summaries while preserving existing user-bubble timestamp behavior.

desktop/src/features/agents/ui/RawEventRail.tsx
Shows the timestamp inside raw activity cards when the preference is on, keeping it visible in collapsed and expanded card states without changing the raw JSON payload.

desktop/src/features/agents/ui/agentSessionTranscriptPresentation.ts
Adds the shared predicate that decides whether a transcript item should receive the new row timestamp.

desktop/src/features/agents/ui/agentSessionTranscriptPresentation.test.mjs
Covers the timestamp visibility rules, including default-off behavior, compact previews, and user-message exclusion.

desktop/src/features/agents/ui/transcriptTimestampPreference.ts
Adds a localStorage-backed device preference for showing transcript row timestamps, matching the existing animation preference pattern.

desktop/src/features/channels/ui/AgentSessionThreadPanel.tsx
Adds the Activity settings menu toggle for Show Timestamps, keeps it available in raw mode, and passes the preference into polished rows and raw cards.

Reproduction Steps

1. Open a channel with an agent Activity pane that has polished activity rows.
2. Open the Activity pane settings menu and confirm Show Timestamps is off by default.
3. Turn Show Timestamps on and confirm muted timestamps appear bottom-left beneath polished activity rows and grouped summaries.
4. Confirm user message bubbles still show only their existing timestamp footer and do not get a duplicate timestamp.
5. Switch to raw activity mode, turn Show Timestamps on, and confirm raw cards show muted timestamps as a second row in both collapsed and expanded states.

Screenshots/Demos

Polished activity rows:

Off (default)	On

Raw mode:

Raw off	Raw on (collapsed)	Raw on (expanded)

Settings menu in Raw mode:

Validation

• pnpm check (biome + file-sizes + px-text)
• pnpm typecheck
• pnpm test — 1586/1586 passing
• Marge screenshot pass: polished timestamps on/off, raw mode timestamps off/on collapsed/expanded, and raw-mode settings menu

Coordination: buzz://message?channel=834029ea-e573-4982-82fd-1f7af039c6c8&thread=22cdeeb1c6372dcf024f1a3409e64ad9b128862647931a85fc425192d014f1ef

[wesbillman]

Nice!

[wesbillman]

Retro review (Brain — PR merged before review, so this is a post-merge glance): clean, no follow-up needed

Pattern-match to transcriptAnimationPreference is exact (module-level useSyncExternalStore store, best-effort localStorage, device-level scope documented). The shouldShowTranscriptRowTimestamp predicate correctly excludes user bubbles (own footer) and compact previews, and it's unit-pinned including the default-off case.

Two trivial notes, neither worth a follow-up PR:

1. The predicate's variant: string param could be typed to the transcript-variant union instead of bare string — as-is a typo'd variant silently behaves like "default".
2. The diff also drops the "Slide new activity rows in…" subtitle from the Show Animations item — intentional menu tightening by the look of it, but it isn't mentioned in the PR body.