feat: add quick theme toggle in sidebar header

[saidMounaim]

What Changed

Added a theme toggle icon button in the sidebar header, next to the T3 Code logo:

• Sun/moon icon that reflects the current resolved theme
• Dropdown menu with System, Light, and Dark options
• Uses existing useTheme hook - syncs automatically with Settings page

Why

Quick theme switching is a common UX pattern. Users shouldn't need to navigate to Settings just to toggle between light and dark mode. This provides convenient access while keeping the full theme configuration in Settings.

UI Changes

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes

Note

Add theme toggle (System/Light/Dark) to sidebar header

Adds a sun/moon icon button to the sidebar header in Sidebar.tsx that opens a dropdown menu for selecting the app theme. The icon reflects the resolved theme (sun for light, moon for dark), and selecting an option calls setTheme from the useTheme hook. Both Electron and non-Electron header layouts are updated to include the toggle.

^{Macroscope summarized 19f9a31.}

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[drj613]

[saidMounaim]

Thanks, the GIF has been re uploaded and should work now

[Noojuno]

I think this clutters up the UI for no real benefit tbh. If you want to change the theme, you can already easily find in the settings

[maria-rcks]

Idk of many people who change themes that often, closing as not planned

@juliusmarminge dk if you have thoughts on this