feat: add tooltips to sidebar and toolbar icons

[davidpoblador]

Problem

First-time users have no way to discover what icons do without clicking them. There are no tooltips on hover for icons in the UI.

Expected behavior

All icon buttons should show a tooltip on hover describing their action (e.g., "New Terminal", "New Browser", "Settings", etc.).

Context

This is a discoverability issue for new users who aren't yet familiar with the icon meanings.

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Reported by @trutx