sweep the dead code in section 8

Each item re-verified before deleting; three of the ten entries were stale
and are corrected in place rather than silently fixed.

- WorkspaceLayout's isMobile/mobilePanelId/onMobileBack: none of its ten
  callers set them, so the mobile collapse they fed was permanently off in
  that renderer. WorkspaceView passes the same props to WorkspaceRenderer
  itself, where they are live.
- fixedHeight on AppRegistryEntry, and getFixedHeight with it: no app has
  ever declared one, so it only contributed undefined. The flex-column
  branch it shared with fitContent stays, keyed on fitContent alone.
- getDefaults: getAllDashboardState already folds the defaults row into the
  one payload the client fetches, which is why it never got a caller.
- upsertScreen's terminals/hostTerminals: never read is right, never
  written was not — it inserted them, which is why all 15 rows hold {}.
  The columns are left in place; dropping them needs a db:push, and this
  tree holds another agent's uncommitted schema file.
- SELECTED_DASHBOARD_KEY: H2 (01365cb) replaced it with ?selected= four
  months ago and it has had no reader since.
- ui/sidebar.tsx and the stray ui/hooks/ beside it. use-mobile was not
  orphaned as claimed — the sidebar imported it — and the use-toast in
  there was a near-identical copy of the live one.
- findChildById's unreachable duplicate condition, and the doc comment that
  described the wrong behaviour rather than the code being wrong.

Left deliberately: DragOverlay/LayoutEditor (gated on 5.3, an owner
decision) and the two chat-owned channels, whose docs are fixed here even
though the publishers are not mine to delete.

CLAUDE.md

@@ -253,8 +253,9 @@ in `usePanelChannel`/`useGlobal`, and has no anchor semantics (no cmd-click, no
 link-focusable). Half the app still does this; none of the new code should.
 
 - **Addressable state goes in the URL** (`useParams` / `useSearchParams`), never in a channel.
-  `usePanelChannel` is for genuine signals and refresh buses (`preview:refresh`,
-  `files:refresh-signal`, `SLSKD_REFRESH_CHANNEL`, `MUSIC_RESYNC_CHANNEL`).
+  `usePanelChannel` is for genuine signals and refresh buses (`files:refresh-signal`,
+  `SLSKD_REFRESH_CHANNEL`, `MUSIC_RESYNC_CHANNEL`) — and declare one with `defineChannel` in
+  `officerdev/src/channels.ts` rather than spelling the name and type at each site.
   "Which thing is open" is a URL. Panels each read the URL rather than passing it between themselves.
 - **Rows and nav items are real links.** `<Link>` for rows (exemplar: the `/chat` session list,
   `f35c145`); **react-router's `<NavLink>`** for nav chrome, so active state comes from the router.