🎨 Palette: Add ARIA labels to icon-only buttons

[threehymns]

This PR adds aria-label attributes to multiple icon-only buttons across the project (src/components/graph-tab.tsx, src/components/frontmatter-editor.tsx, src/components/editor/extensions/code-block-codemirror.tsx).

What

Added explicit aria-labels to buttons where only a title was present or no text context was provided.

Why

Screen readers do not reliably read title attributes on interactive elements, which creates accessibility issues for users relying on assistive technology. Explicit aria-labels improve UX for visually impaired users.

Accessibility

Greatly improves a11y for core micro-actions like resetting settings, closing panels, removing groups, toggling line wrap, and copying code block content.

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PR created automatically by Jules for task 16228587504763397939 started by @threehymns

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