feat: implement UX enhancements for playground

Add comprehensive UX improvements to color playground:

**URL State Sharing:**
- Implement URL parameter sync for colors
- Add share button to copy playground URLs
- Wrap in Suspense boundary for Next.js 16 compatibility
- Colors persist in URL for easy sharing

**Keyboard Shortcuts:**
- Create reusable useKeyboard hook with modifier support
- Add Cmd/Ctrl+C to copy current color
- Add Cmd/Ctrl+S to share color URL
- Add Cmd/Ctrl+R for random color generation
- Display keyboard hints in playground header
- Cross-platform support (Mac/Windows/Linux)

**Color History:**
- Implement Zustand store with localStorage persistence
- Track up to 50 most recent colors with timestamps
- Auto-deduplicate and keep most recent entry
- Add visual history grid in playground
- Click to restore previous colors
- Individual color removal with hover UI
- Clear all history option

**Playground UI Enhancements:**
- Add visual keyboard shortcut indicators (⌘C, ⌘S, ⌘R)
- Implement Recent Colors section with 5-column grid
- Add hover effects for history color swatches
- Individual remove buttons on hover
- Toast notifications for all actions
- Improved button layout and spacing

**Files Added:**
- lib/hooks/useKeyboard.ts - Keyboard shortcut management
- lib/stores/historyStore.ts - Color history with persistence

**Files Modified:**
- app/playground/page.tsx - Integrated all UX features

Successfully builds with all features working.
Ready for user testing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/stores/historyStore.ts

@@ -0,0 +1,68 @@
+import { create } from 'zustand';
+import { persist, createJSONStorage } from 'zustand/middleware';
+
+export interface ColorHistoryEntry {
+  color: string;
+  timestamp: number;
+}
+
+interface ColorHistoryState {
+  history: ColorHistoryEntry[];
+  addColor: (color: string) => void;
+  removeColor: (color: string) => void;
+  clearHistory: () => void;
+  getRecent: (limit?: number) => ColorHistoryEntry[];
+}
+
+/**
+ * Color history store with localStorage persistence
+ *
+ * Tracks up to 50 most recent colors with timestamps
+ * Automatically removes duplicates (keeps most recent)
+ * Persists across browser sessions
+ */
+export const useColorHistory = create<ColorHistoryState>()(
+  persist(
+    (set, get) => ({
+      history: [],
+
+      addColor: (color) => {
+        const normalizedColor = color.toLowerCase();
+        set((state) => {
+          // Remove existing entry if present
+          const filtered = state.history.filter(
+            (entry) => entry.color.toLowerCase() !== normalizedColor
+          );
+
+          // Add new entry at the beginning
+          const newHistory = [
+            { color: normalizedColor, timestamp: Date.now() },
+            ...filtered,
+          ].slice(0, 50); // Keep only 50 most recent
+
+          return { history: newHistory };
+        });
+      },
+
+      removeColor: (color) => {
+        const normalizedColor = color.toLowerCase();
+        set((state) => ({
+          history: state.history.filter(
+            (entry) => entry.color.toLowerCase() !== normalizedColor
+          ),
+        }));
+      },
+
+      clearHistory: () => set({ history: [] }),
+
+      getRecent: (limit = 10) => {
+        const { history } = get();
+        return history.slice(0, limit);
+      },
+    }),
+    {
+      name: 'pastel-color-history',
+      storage: createJSONStorage(() => localStorage),
+    }
+  )
+);