Improve accessibility: add ARIA labels to interactive elements

[yandanp]

Description

Several interactive elements (buttons, tabs, panels) are missing proper ARIA labels, making the app less accessible for screen reader users.

Task

Audit and add appropriate aria-label, role, and aria-* attributes to:

• Sidebar buttons
• Tab bar (tabs, close buttons, add button)
• Settings modal
• Shell picker dropdown

Tech

• Frontend only (React/TypeScript)
• Components are in src/renderer/components/

Resources

• WAI-ARIA Authoring Practices

Getting Started

See CONTRIBUTING.md for setup instructions.