feat(console): selected_at anchor recovers item window on stale shared link

[vaderyang]

Stacked on top of #18 — base is `feat/selected-id-in-url`. The diff against `main` here shows only the anchor work.

Why

Copying a list URL like `?preset=15m&selected=` and opening it half an hour later: `start=now-15m, end=now` is computed from scratch, the selected item is no longer in that window. The detail panel still loaded (queries by id) but the list behind it was unrelated, the row had no highlight, prev/next disabled. Mid-experience drop.

Approach

Pin the item's wall-clock time alongside the id:

1. List pages write `?selected_at=<unix_s>` when an item is selected — from `start_time` (agent turns, microseconds) or `request_time` (llm calls / http exchanges, milliseconds). Cleared together with `selected` on close.

2. `useToolbarUrlSync` hydration reads the anchor. If it falls outside the preset-derived window:

   • keep the preset's duration (the original user's "show me this much context" signal),
   • slide so `end = anchor + 60s` (small pad — keeps the item from sitting flush at the edge in a desc-by-time list),
   • promote `preset` to `custom` so subsequent URL writes carry absolute `start`/`end`.

   No-op when the anchor is inside the window, absent, unparseable, or already covered.

3. Pure helper `applySelectedAtAnchor` in its own module (`selected-at-anchor.ts`, no `@/` aliases) so it's directly testable under bun. 7 unit tests:

   • no-op: missing param, unparseable param, anchor inside window
   • shift: 15m window, anchor 30 min ago → custom window of 15m ending at anchor+60s
   • default 1h window when no preset in URL
   • 1h duration preserved across an arbitrary shift
   • clock-skew / future-dated anchor

Behaviour summary

Scenario	Before	After
Original tab clicking around (preset=15m)	Toolbar stays relative, ticks every refresh	Same — anchor only affects fresh URL loads
Recipient opens shared URL within window	Works	Same
Recipient opens shared URL after window moved	Panel opens but list is empty / unrelated	Window auto-widens to bracket the item; list, highlight, prev/next all work
Recipient opens URL with anchor older than retention	Panel 404s	Window slides correctly but item itself is gone (no help possible)

Verification

• `bun test` — 104 pass (7 new for the anchor logic)
• `npm run build` — green
• Deployed to wuneng (bundle index-ZiliosRk.js)

Test plan

• Open a row in Agent Turns → URL now contains `?selected=&selected_at=<unix_s>`
• Wait > 15 minutes
• Open the same URL in a new tab → list re-anchors around the item, panel opens, prev/next active
• Same flow for LLM Calls and HTTP Exchanges
• Original tab: switching presets still snaps to `now` (not `custom`) — confirm the anchor doesn't leak into post-mount behaviour

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