Flatten settings sidebar with a host picker (#1241)

* Flatten settings sidebar with a host picker

Settings had two implicit tiers: app-level sections as flat sidebar rows,
and per-host settings hidden behind a drill-in to one monolithic host page.
This flattens both into a single sidebar with an "App" group and a "Host"
group, where the host's settings are top-level rows scoped by a host picker.

- Split the monolithic HostPage into four section pages: Connections,
  Orchestration (was the unnamed inject-tools + system-prompt block),
  Providers, and Daemon (status, updates, restart, remove).
- Host picker reuses the canonical <Combobox> sidebar host switcher; defaults
  to the local host so a single-local-host user never thinks about "the daemon".
- Host sections become real sub-routes (/settings/hosts/[id]/[section]); the
  old /settings/hosts/[id] redirects to the connections section, so existing
  deep links (pairing, workspace, open-project) keep working.
- "Open providers" from the project screen now lands on the Providers section.

* Fix e2e host-settings navigation for the flat sidebar

The flat-settings refactor moved host settings onto separate section pages,
but three e2e helpers still expected the old monolithic host page:

- openDesktopSettings: the daemon-lifecycle card now lives on the Daemon
  section — navigate there before asserting it.
- acp catalog: "Add provider" now lives on the Providers section — open that
  section before opening the add-provider modal.
- openAddHostFlow: "Add host" is now an item inside the host picker (Combobox),
  not a standalone button — open the picker first.

Test-only; no product changes. Fixes the 8 playwright failures on this branch.

packages/app/e2e/acp-provider-catalog.spec.ts

@@ -6,6 +6,7 @@ import {
   installAcpCatalogProvider,
   openAddProviderModal,
   openSettingsHost,
+  openSettingsHostSection,
 } from "./helpers/settings";
 
 const ACP_PROVIDER = {
@@ -18,6 +19,8 @@ test.describe("ACP provider catalog", () => {
     await gotoAppShell(page);
     await openSettings(page);
     await openSettingsHost(page, getServerId());
+    // Providers moved to their own host section; add-provider lives there now.
+    await openSettingsHostSection(page, getServerId(), "providers");
     await openAddProviderModal(page);
 
     await installAcpCatalogProvider(page, ACP_PROVIDER.name);