fix(desktop): anchor active-turn badge to skew-corrected agent start

[wpfleger96]

What

The active-turn badge elapsed counter anchored to Date.now() at the moment the desktop processed the turn event. Two consequences:

• Turns processed in the same JS tick (observer buffer replay on first subscribe, or several turns in one syncAll()) all received the same Date.now() anchor, so their timers ticked in perfect lockstep — the user-reported "two timers at exactly the same number, in sync" symptom.
• A turn that actually started minutes earlier rendered 0s the instant the desktop first observed it.

Why this approach

Switching the anchor to the raw agent-host startedAt was off the table — it reintroduces agent-host/desktop clock skew into the elapsed display, which the original Date.now() anchor existed to prevent.

Instead the store maintains a per-agent clock offset = running minimum of Date.now() - Date.parse(event.timestamp), sampled on every fresh observer event. The badge anchor is derived at read time in getActiveTurnsForAgent as startedAt + offset(agent):

• Distinct agent-host start times produce distinct anchors → no lockstep.
• A long-running turn shows its true elapsed immediately.
• Skew is corrected without trusting the raw agent clock; the running-minimum estimator converges on true skew minus minimum network delay, so the badge never inflates elapsed (it under-reports by at most the smallest observed network delay).
• Read-time derivation means a later, tighter offset retroactively corrects every live badge — freezing the anchor at insert would cancel that correction.

The ActiveTurnSummary.observedAt field is renamed to anchorAt to match the new semantics; the three consumers (UserProfilePanelSections, UserProfilePopover, ManagedAgentRow) get the identical mechanical rename and nothing else.

Scope

5 files, +263/-73. Logic + tests in activeAgentTurnsStore.{ts,test.mjs}; pure field rename in the three consumers.