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/src/utils/host-routes.ts

@@ -390,6 +390,14 @@ export function isSettingsSectionSlug(value: string): value is SettingsSectionSl
   return (SETTINGS_SECTION_SLUGS as readonly string[]).includes(value);
 }
 
+export const HOST_SECTION_SLUGS = ["connections", "orchestration", "providers", "daemon"] as const;
+
+export type HostSectionSlug = (typeof HOST_SECTION_SLUGS)[number];
+
+export function isHostSectionSlug(value: string): value is HostSectionSlug {
+  return (HOST_SECTION_SLUGS as readonly string[]).includes(value);
+}
+
 export function buildSettingsRoute() {
   return "/settings" as const;
 }
@@ -406,6 +414,14 @@ export function buildSettingsHostRoute(serverId: string) {
   return `/settings/hosts/${encodeSegment(normalized)}` as const;
 }
 
+export function buildSettingsHostSectionRoute(serverId: string, section: HostSectionSlug) {
+  const normalized = trimNonEmpty(serverId);
+  if (!normalized) {
+    throw new Error("buildSettingsHostSectionRoute requires a non-empty serverId");
+  }
+  return `/settings/hosts/${encodeSegment(normalized)}/${section}` as const;
+}
+
 export function buildProjectsSettingsRoute() {
   return "/settings/projects" as const;
 }