move terminal-verbosity toggle from agent overflow menu to /settings.html

Per mara: 'i wanted you to put this in .../settings.html' — the toggle
belongs with the other operator-local browser preferences, not buried
in each agent's own overflow menu.

Extracted the get/set + localStorage key into @hive/shared/prefs.js so
settings.html (writer) and every per-agent app.js (reader, via
HiveTerminal.create's expandDetails option) agree on the exact same key
without two independently-typed copies that could drift. Removed the
now-unused overflow-menu toggle + its agent.css rules from the agent
page. Docs moved from docs/web-ui/agent.md's overflow-button section to
docs/web-ui/dashboard.md's S3TT1NGS section, next to the existing
browser-notifications preference.

docs/web-ui/agent.md

@@ -101,16 +101,6 @@ through. Three flex columns:
     the `/effort <level>` slash command). The active level's button is
     highlighted; `renderEffortChip` keeps the picker in sync with
     `StateSnapshot.effort` from the cold-load snapshot.
-    A third separator + **settings** section holds a single
-    `role="menuitemcheckbox"` toggle, `☐/☑ expand tool output` — flips
-    whether otherwise-collapsed `<details>` rows in the terminal (long
-    tool-results, Write/Edit diffs, …) default open. Pure client-side:
-    persisted to this browser's `localStorage` only (no backend field,
-    no `/api/*` call), read live by the shared terminal factory's
-    `expandDetails` option (see docs/web-ui/shape.md::Shared terminal
-    pane) so toggling mid-session applies to the next rendered row
-    without a reload. Rows that already default open regardless (send /
-    ask / answer / recv) are unaffected either way.
     The popover's display rules are scoped to `:not([hidden])` so the
     `[hidden]` HTML attribute's UA `display: none` isn't overridden by
     the author CSS's `display: flex` — the popover stays hidden until