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/layout.css

@@ -202,15 +202,17 @@
     align-items: center;
     justify-content: center;
     margin-left: 6px;
-    min-width: 16px;
-    height: 16px;
-    padding: 0 3px;
+    min-width: 18px;
+    height: 18px;
+    padding: 0 5px;
     font-size: 11px;
     font-weight: 600;
     line-height: 1;
+    text-transform: uppercase;
     border-radius: var(--border-radius-xs);
     background-color: var(--shortcut-bg);
     border: 1px solid var(--shortcut-border);
+    box-shadow: var(--shortcut-shadow);
     color: var(--shortcut-text);
     vertical-align: middle;
     opacity: 0.8;
@@ -219,12 +221,8 @@
 
 .control-group button:hover .shortcut-key {
     opacity: 1;
-    background-color: oklch(var(--lora-accent-l) var(--lora-accent-c) var(--lora-accent-h) / 0.2);
-}
-
-[data-theme="dark"] .shortcut-key {
-    --shortcut-bg: oklch(var(--lora-accent-l) var(--lora-accent-c) var(--lora-accent-h) / 0.15);
-    --shortcut-border: oklch(var(--lora-accent-l) var(--lora-accent-c) var(--lora-accent-h) / 0.3);
+    background-color: var(--shortcut-bg-hover);
+    border-color: var(--shortcut-border-hover);
 }
 
 /* Ensure correct vertical alignment for text+shortcut */