Add light/dark mode theme support with OS sync

- Added theme state management (light/dark/system)
- Implemented OS theme detection and automatic sync
- Added theme toggle UI in sidebar with 3 buttons
- Applied dark mode styles to all components:
  - Sidebar with dark backgrounds and borders
  - Note editor with dark text and inputs
  - Toolbar buttons with dark hover states
  - TipTap editor with dark mode text and code blocks
- Theme preference saved to localStorage
- Enabled Tailwind dark mode with class strategy
- Smooth transitions between themes

src/index.css

@@ -19,6 +19,11 @@ code {
 /* TipTap Editor Styles */
 .ProseMirror {
   min-height: 100%;
+  color: #111827;
+}
+
+.dark .ProseMirror {
+  color: #f3f4f6;
 }
 
 .ProseMirror:focus {
@@ -72,6 +77,12 @@ code {
   border-radius: 0.25rem;
   font-family: 'Courier New', monospace;
   font-size: 0.9em;
+  color: #1f2937;
+}
+
+.dark .ProseMirror code {
+  background-color: #374151;
+  color: #f3f4f6;
 }
 
 .ProseMirror pre {
@@ -83,6 +94,10 @@ code {
   margin: 1em 0;
 }
 
+.dark .ProseMirror pre {
+  background-color: #111827;
+}
+
 .ProseMirror pre code {
   background-color: transparent;
   padding: 0;