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/components/Help/HelpButton.tsx

@@ -0,0 +1,32 @@
+import React from 'react'
+import { HelpCircle } from 'lucide-react'
+import { useTranslation } from '../../i18n'
+import { useHelpStore } from '../../store/helpStore'
+
+/**
+ * The `?` in the navbar. Same 32px footprint as the theme toggle beside it;
+ * lit while the panel is open so the two read as one control.
+ */
+export default function HelpButton({ className }: { className?: string } = {}): React.ReactElement {
+  const { t } = useTranslation()
+  const open = useHelpStore(s => s.open)
+  const toggleHelp = useHelpStore(s => s.toggleHelp)
+  return (
+    <button
+      type="button"
+      onClick={toggleHelp}
+      title={t('help.center.button')}
+      aria-label={t('help.center.button')}
+      aria-pressed={open}
+      data-help-button
+      className={
+        className ??
+        `hidden sm:flex relative w-8 h-8 p-2 rounded-lg items-center justify-center flex-shrink-0 transition-colors ${
+          open ? 'bg-surface-selected text-content' : 'text-content-muted hover:bg-surface-hover'
+        }`
+      }
+    >
+      <HelpCircle className="w-4 h-4" />
+    </button>
+  )
+}