feat: UX improvements — copy buttons, shortcuts modal, breadcrumb rewind, create tree CTA

- Add copy-to-clipboard buttons on command blocks (action + custom step nodes)
- Add keyboard shortcuts modal (?) with option loading spinners and [Esc] hint
- Restyle session timer as a pill badge for better visibility
- Add prominent "Create Tree" CTA to MyTreesPage header and empty state
- Make breadcrumb items clickable to rewind navigation to any previous step

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/hooks/useKeyboardShortcuts.ts

@@ -51,6 +51,7 @@ export interface TreeNavigationShortcutsConfig {
   onSelectOption: (index: number) => void
   onGoBack: () => void
   onContinue: () => void
+  onShowShortcuts?: () => void
   optionCount: number
   canGoBack: boolean
   canContinue: boolean
@@ -60,6 +61,7 @@ export function useTreeNavigationShortcuts({
   onSelectOption,
   onGoBack,
   onContinue,
+  onShowShortcuts,
   optionCount,
   canGoBack,
   canContinue,
@@ -89,6 +91,13 @@ export function useTreeNavigationShortcuts({
         document.getElementById('session-notes')?.focus()
       },
     },
+    // ? to show shortcuts modal
+    {
+      key: '?',
+      shift: true,
+      handler: () => onShowShortcuts?.(),
+      enabled: !!onShowShortcuts,
+    },
   ]
 
   useKeyboardShortcuts(shortcuts)