agent web UI: add terminal verbosity setting (expand tool output by default)

Adds a browser-local (localStorage only, no backend field) toggle in
the per-agent overflow menu's new settings section: whether otherwise-
collapsed <details> rows in the live terminal (long tool-results,
Write/Edit diffs, ...) default open. Message-bearing rows that already
default open (send/ask/answer/recv) are unaffected either way.

The shared terminal factory (frontend/packages/shared/src/terminal/terminal.js)
gains an optional expandDetails option (boolean or zero-arg function),
read live on every details()/detailsDiff() call rather than captured
once, so flipping the toggle mid-session applies to the next rendered
row without a reload. Unused by the dashboard's own terminal pane, so
its default-closed behaviour is unchanged.

Closes #2961.

docs/web-ui/shape.md

@@ -115,7 +115,11 @@ event picture), `onBackfillDone?(count)` (one-shot after replay; `count=0` on
 failed/skipped fetch), `onStreamOpen?()` (fires on every EventSource
 (re)connect — use to re-sync snapshot-derived state after a reconnect gap),
 `pillAnchor?` (parent element for the "↓ N new" pill; defaults to
-`logEl.parentElement`).
+`logEl.parentElement`), `expandDetails?` (boolean or zero-arg function
+returning one, re-read on every `api.details`/`api.detailsDiff` call rather
+than captured once — lets a page default otherwise-collapsed panels open
+per a live browser-local preference; renderers that force a row open
+regardless, e.g. message-bearing tool_use, are unaffected either way).
 
 **Sticky-bottom + snap animation.** `stickToBottom` is the
 operator's intent: true means "keep snapping to bottom on every