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/index.css

@@ -52,12 +52,10 @@
   background: #94a3b8;
 }
 
-/* Smooth transitions */
-* {
-  transition-property: background-color, border-color, color, opacity, box-shadow, transform;
-  transition-duration: 200ms;
-  transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1);
-}
+/* Smooth transitions — scoped to interactive elements only.
+   Do NOT use * selector — it causes every element to re-animate
+   on any React state change (e.g. drag-and-drop). Components should
+   use Tailwind transition-colors / transition-all where needed. */
 
 /* Arabic text support */
 [dir="rtl"] {
@@ -334,14 +332,7 @@ textarea {
 /* Refined button styles */
 button {
   border-radius: 0.625rem;
-  transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
-}
-button:hover:not(:disabled) {
-  transform: translateY(-1px);
-  box-shadow: 0 4px 12px -2px rgba(0, 0, 0, 0.12);
-}
-button:active:not(:disabled) {
-  transform: translateY(0) scale(0.98);
+  transition: background-color 0.2s, border-color 0.2s, color 0.2s, box-shadow 0.2s;
 }
 button:disabled {
   cursor: not-allowed;
@@ -386,11 +377,6 @@ select:not(:disabled):hover {
   color: white;
 }
 
-/* Kanban column */
-.kanban-column {
-  min-height: 200px;
-}
-
 /* Calendar grid */
 .calendar-grid {
   display: grid;