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)

tests/frontend/components/pageControls.randomSort.test.js

@@ -13,6 +13,12 @@ vi.mock('../../../static/js/utils/uiHelpers.js', () => ({
   showToast: vi.fn(),
   openCivitaiByMetadata: vi.fn(),
   updatePanelPositions: vi.fn(),
+  // Faithful stand-in for the real helper in uiHelpers.js
+  isTypingContext: (target) => {
+    if (!(target instanceof Element)) return false;
+    const tagName = target.tagName?.toLowerCase();
+    return target.isContentEditable || tagName === 'input' || tagName === 'textarea' || tagName === 'select';
+  },
 }));
 
 vi.mock('../../../static/js/managers/DownloadManager.js', () => ({