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)

locales/en.json

@@ -1977,6 +1977,7 @@
       "title": "Keyboard & Mouse Shortcuts",
       "groups": {
         "general": "General",
+        "actions": "Actions",
         "selection": "Selection & Bulk Mode",
         "navigation": "Navigation",
         "modelModal": "Model / Recipe Modal",
@@ -1993,6 +1994,9 @@
         "focusSearch": "Focus search",
         "closeModal": "Close modal / panel",
         "openShortcuts": "Open this shortcuts panel",
+        "refresh": "Refresh model list",
+        "fetchMetadata": "Fetch metadata from CivitAI (model pages only)",
+        "downloadModel": "Download a model (model pages only)",
         "toggleBulkMode": "Toggle bulk mode",
         "selectAll": "Select all visible models",
         "rangeSelect": "Range select",