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/contexts/ThemeContext.jsx

@@ -0,0 +1,38 @@
+import { createContext, useContext, useState, useEffect } from 'react'
+
+const ThemeContext = createContext()
+
+export function ThemeProvider({ children }) {
+  const [darkMode, setDarkMode] = useState(() => {
+    // Check localStorage or system preference
+    const stored = localStorage.getItem('darkMode')
+    if (stored !== null) return stored === 'true'
+    return window.matchMedia('(prefers-color-scheme: dark)').matches
+  })
+
+  useEffect(() => {
+    // Apply dark mode class to document
+    if (darkMode) {
+      document.documentElement.classList.add('dark')
+    } else {
+      document.documentElement.classList.remove('dark')
+    }
+    localStorage.setItem('darkMode', String(darkMode))
+  }, [darkMode])
+
+  const toggleDarkMode = () => setDarkMode(prev => !prev)
+
+  return (
+    <ThemeContext.Provider value={{ darkMode, toggleDarkMode }}>
+      {children}
+    </ThemeContext.Provider>
+  )
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext)
+  if (!context) {
+    throw new Error('useTheme must be used within ThemeProvider')
+  }
+  return context
+}