feat: add theme toggle, shared KanbanCard, keyboard shortcuts

Previously unstaged files from prior sessions: ThemeContext,
ThemeToggle, KanbanCard, useKeyboardShortcuts hook, and updated
Header, KanbanBoard, Issues, Tasks, PostProduction, index.css.

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

client/src/components/Header.jsx

@@ -4,6 +4,7 @@ import { Bell, ChevronDown, LogOut, Shield, Lock, AlertCircle, CheckCircle } fro
 import { useAuth } from '../contexts/AuthContext'
 import { getInitials, api } from '../utils/api'
 import Modal from './Modal'
+import ThemeToggle from './ThemeToggle'
 
 const pageTitles = {
   '/': 'Dashboard',
@@ -98,7 +99,10 @@ export default function Header() {
       </div>
 
       {/* Right side */}
-      <div className="flex items-center gap-4">
+      <div className="flex items-center gap-2">
+        {/* Theme toggle */}
+        <ThemeToggle />
+
         {/* Notifications */}
         <button className="relative p-2 rounded-lg hover:bg-surface-tertiary text-text-secondary hover:text-text-primary transition-colors">
           <Bell className="w-5 h-5" />