feat: align workspace sidebar and Explorer layout

[lalondesteve]

I’ve aligned the Workspaces sidebar and Explorer for a more consistent look, with matching row sizing, spacing, and headers.

The mobile sidebar now has a top-right close button that stays visible on narrow screens. Desktop resizing and existing navigation behavior are preserved.

Checked on desktop and mobile-sized layouts, including larger controls and reduced motion.

[andrebrait]

Thanks for this! Before we go further: have you tried the mobile sidebar that is already in main? #208 merged shortly before this PR was opened and already makes the workspace sidebar full-screen on phones, with a close button in the top-left corner.

We also added gesture support in #208: on mobile, swiping right anywhere on the app surface opens the left sidebar (or closes the file panel), and swiping left does the opposite. Taps, vertical scrolling, text editing and open popups keep their normal behavior.

If, after trying it, you still feel that moving the X button to the top-right, hiding the back/forward buttons on narrow phones, or the other sidebar changes here are important, could you let us know which ones and why? That will help us decide what to keep.

[lalondesteve]

Sorry, I missed this PR, I'll retract this one and try yours first!