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.

docs/navigation-audit.md

@@ -123,10 +123,15 @@ This is the primary surface to convert to URL-driven selection.
 | settings `useGlobal(*_SELECTED)` | settings section | `/settings/:page/:section` | `SettingsPanel.tsx:56` (M6) |
 | local `useState` selections | search / room / conversation / peer / open file / preview slug | respective nested routes | `SearchView`, `SoulseekRooms`, `SoulseekChat`, `SoulseekUsers`, `CodeEditor/useEditorState`, `PreviewProvider` |
 
-**Excluded — event-bus / refresh signals, NOT selection:** `preview:refresh`, `files:refresh-signal`,
-`SLSKD_REFRESH_CHANNEL`, `MUSIC_RESYNC_CHANNEL`, `chat:active-session` (embedded-chat signal).
+**Excluded — event-bus / refresh signals, NOT selection:** `files:refresh-signal`,
+`SLSKD_REFRESH_CHANNEL`, `MUSIC_RESYNC_CHANNEL`.
 (`FILE_VIEWER_CHANNEL` was listed here too; it had no publisher and has been deleted — the file viewer
-reads `?view=` from the URL.)
+reads `?view=` from the URL. `preview:refresh` and `chat:active-session` were also listed, and
+`preview:refresh` was cited above as the exemplar of a *legitimate* channel — but both have a publisher
+in `ChatPanelWrapper` and **no subscriber at all**, and `preview:refresh`'s reader, `PreviewProvider`, is
+no longer in the repo. They are declared in `officerdev/src/channels.ts` with that stated; deleting the
+publishers means changing the chat panel, which is another agent's, so it is written up in
+`COMMS/chat-agent-handoff-2026-08-07.md` instead. Use `files:refresh-signal` as the exemplar.)
 
 ---
 
@@ -177,7 +182,7 @@ reads `?view=` from the URL.)
 
 ### Phase 1 — Quick wins (routes already exist; mechanical, high value)
 - [x] **H1** Jobs rows → `<Link to={`/jobs/${job.id}`}>` (`JobsPage.tsx`). Done — `46482f3`. (active-row highlight already keyed off `useParams().id`.)
-- [x] **H2** Dashboards rows → `<Link>`; `SELECTED_DASHBOARD_KEY` global replaced by `?selected=` URL param across `DashboardListApp`/`DashboardsScreen`/`DashboardPreview`. Done — `01365cb`. **Needs runtime test.**
+- [x] **H2** Dashboards rows → `<Link>`; `SELECTED_DASHBOARD_KEY` global replaced by `?selected=` URL param across `DashboardListApp`/`DashboardsScreen`/`DashboardPreview`. Done — `01365cb`. **Needs runtime test.** (The constant itself outlived its last reader by four months and has now been deleted; its siblings in `Dashboards/constants.ts` are dialog form state, not selection, and stay.)
 - [x] **H3** Projects rows → `<Link>`; `SELECTED_PROJECT` global replaced by `?selected=` URL param across `ProjectListApp`/`ProjectListScreen`/`ProjectPreview`. Done — `2aaacc8`. **Needs runtime test.**
 - [ ] **H4** Chat detail: read `sessionId` from `useParams`, retire `chat:selected-session` as source of truth (`ChatDetailPanel.tsx:136`) — **finishes the /chat fix**. *Deferred: overlaps the in-flight `sidecars-*` chat-comms work; do after that lands.*
 - [ ] **H5** Email rows → `<Link>` driven by `useParams().emailId`; delete the state↔URL sync effects (`EmailList.tsx:336`, `EmailScreen.tsx:27-33`).