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",

frontend/src/core/i18n/locales/types.ts

@@ -211,6 +211,17 @@ export interface Translations {
     failed: string;
   };
 
+  // Shortcuts
+  shortcuts: {
+    searchActions: string;
+    noResults: string;
+    actions: string;
+    keyboardShortcuts: string;
+    keyboardShortcutsDescription: string;
+    openCommandPalette: string;
+    toggleSidebar: string;
+  };
+
   // Settings
   settings: {
     title: string;

frontend/src/core/i18n/locales/zh-CN.ts

@@ -261,6 +261,17 @@ export const zhCN: Translations = {
     failed: "子任务失败",
   },
 
+  // Shortcuts
+  shortcuts: {
+    searchActions: "搜索操作...",
+    noResults: "未找到结果。",
+    actions: "操作",
+    keyboardShortcuts: "键盘快捷键",
+    keyboardShortcutsDescription: "使用键盘快捷键更快地操作 DeerFlow。",
+    openCommandPalette: "打开命令面板",
+    toggleSidebar: "切换侧边栏",
+  },
+
   // Settings
   settings: {
     title: "设置",