feat(ui): add R/F/D action shortcuts and unify keycap hint style

- Bind R=refresh, F=fetch metadata, D=download in PageControls via
  eventManager (plain letters only, skipped while typing or when a
  modal is open); triggers reuse the buttons' existing click handlers
- Show key-hint chips on the refresh/fetch/download/bulk toolbar
  buttons; convert the bulk chip to a semantic <kbd>
- Redesign shortcut hints as a neutral theme-adaptive keycap:
  --shortcut-* variables in base.css now derive from --text-muted
  with a bottom-edge shadow, shared by the toolbar chips, the header
  search cue, the help-modal cheat sheet, and onboarding key hints
- Add shared isTypingContext() helper to uiHelpers
- Add an Actions group (R/F/D) to the Shortcuts cheat-sheet tab

Verified with vitest (926 passing, incl. 6 new shortcut cases) and a
sandboxed E2E run in real Chrome (light/dark rendering, hover state,
'?' opening the Shortcuts tab, clean console)

static/css/onboarding.css

@@ -205,6 +205,7 @@
     display: inline-block;
     background: var(--shortcut-bg);
     border: 1px solid var(--shortcut-border);
+    box-shadow: var(--shortcut-shadow);
     border-radius: var(--border-radius-xs);
     padding: 2px 6px;
     font-size: 0.8em;