🎨 Palette: Add missing ARIA labels to icon-only buttons

[khangnghiem]

💡 What

Added aria-label attributes exclusively to icon-only buttons (like tool selectors, undo/redo, etc.) in site/index.html.

🎯 Why

Icon-only buttons are completely opaque to screen readers without an aria-label. This was a major accessibility gap in the web application's UI, especially for core interaction tools.

📸 Before/After

• Before: <button class="ft-tool-btn" data-tool="hand"><svg>...</svg></button>
• After: <button class="ft-tool-btn" data-tool="hand" aria-label="Hand"><svg>...</svg></button>

♿ Accessibility

This change ensures that all primary interaction tools on the canvas (Hand, Select, Rectangle, Ellipse, Pen, Arrow, Text, Eraser, Lasso, Undo, Redo, AI Touch) are now discoverable and clearly announced to screen reader users, adhering to WCAG guidelines for non-text content. Care was taken to NOT add redundant aria-labels to buttons that already have visible text.

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PR created automatically by Jules for task 2898699611078508689 started by @khangnghiem

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