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/hooks/useHelpContext.ts

@@ -0,0 +1,18 @@
+import { useEffect } from 'react'
+import { useHelpStore } from '../store/helpStore'
+
+/**
+ * Tell the help center which screen is showing. Call it from the screen's data
+ * hook, or render `<HelpAnchor id>` from a page body; either way the context
+ * is set while the screen is mounted and cleared when it goes away, unless
+ * another screen has already taken over.
+ */
+export function useHelpContext(id: string): void {
+  const setContext = useHelpStore(s => s.setContext)
+  useEffect(() => {
+    setContext(id)
+    return () => {
+      if (useHelpStore.getState().contextId === id) setContext(null)
+    }
+  }, [id, setContext])
+}