feat(help): in-app help center with per-screen guides and pictures

A help icon on every screen opens a panel with the screen's overview, its
sub-screens, task guides with a framed picture per step, search across guides
and the wiki, and a switch to any other screen's help. Guides exist for the
dashboard, Vacay, Atlas, Collections, Journey (list, journal, studio), Settings
(all tabs), Admin (all tabs) and the trip frame; the trip's own screens are
in progress. Pictures are produced by Playwright (client/e2e/help), texts in
all 23 languages.

client/src/api/help.ts

@@ -0,0 +1,23 @@
+import { apiClient } from './client'
+
+/** One hit of the full-text search over the bundled wiki (`GET /api/help/search`). */
+export interface HelpSearchHit {
+  slug: string
+  title: string
+  section: string
+  anchor: string | null
+  heading: string | null
+  snippet: string
+  score: number
+}
+
+/**
+ * Search the docs. Online-only by nature: the pages live on the server, and a
+ * help lookup is never queued for later. Pass a signal so a superseded query
+ * can be dropped instead of racing the newer one.
+ */
+export function searchHelp(query: string, signal?: AbortSignal, limit = 8): Promise<HelpSearchHit[]> {
+  return apiClient
+    .get<{ hits: HelpSearchHit[] }>('/help/search', { params: { q: query, limit }, signal })
+    .then(r => r.data.hits)
+}