Add settings gear icon to sidebar (task 13.2)

Added a Settings icon button next to the theme toggle in the sidebar header
that links to /app/settings. This provides quick access to user settings
from the main workspace.

- Import Link from next/link in app/page.tsx
- Add Settings icon with link to /app/settings in sidebar header
- Mark task 13.2 as complete in tasks.md

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

src/app/app/page.tsx

@@ -1,6 +1,7 @@
 'use client';
 
 import { useState, useRef, useEffect, useCallback } from 'react';
+import Link from 'next/link';
 import Toolbox, { Tool } from '@/components/Toolbox';
 import FileUpload from '@/components/FileUpload';
 import CandleChart, { CandleChartHandle } from '@/components/CandleChart';
@@ -833,6 +834,13 @@ export default function Home() {
             <p className="text-[10px] text-muted-foreground">Chart annotation tool</p>
           </div>
           <div className="flex items-center gap-1">
+            <Link
+              href="/app/settings"
+              className="p-1.5 rounded hover:bg-secondary/50 transition-colors text-muted-foreground hover:text-foreground"
+              title="Settings"
+            >
+              <Settings className="w-4 h-4" />
+            </Link>
             <ThemeToggle />
           </div>
         </div>