feat(frontend): add Cmd+K command palette and keyboard shortcuts (#1230)

* feat(frontend): add Cmd+K command palette and keyboard shortcuts

Wire up the existing shadcn/ui Command component as a global command
palette. Adds a useGlobalShortcuts hook for Cmd+K (palette), Cmd+Shift+N
(new chat), Cmd+, (settings), and Cmd+/ (shortcuts help overlay).

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

* fix(frontend): address Copilot review feedback on command palette

- Normalize event.key with toLowerCase() for reliable Shift+key matching
- Replace dead deerflow:open-settings event with router.push navigation
- Use platform-appropriate Shift label (Shift+ on Windows/Linux, glyph on Mac)

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

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/i18n/locales/en-US.ts

@@ -274,6 +274,17 @@ export const enUS: Translations = {
     failed: "Subtask failed",
   },
 
+  // Shortcuts
+  shortcuts: {
+    searchActions: "Search actions...",
+    noResults: "No results found.",
+    actions: "Actions",
+    keyboardShortcuts: "Keyboard Shortcuts",
+    keyboardShortcutsDescription: "Navigate DeerFlow faster with keyboard shortcuts.",
+    openCommandPalette: "Open Command Palette",
+    toggleSidebar: "Toggle Sidebar",
+  },
+
   // Settings
   settings: {
     title: "Settings",