fix(theme): make native select popups follow dark theme on Linux

[Nic-dorman]

Summary

• Adds color-scheme: dark to :root (and color-scheme: light to :root.light) in assets/css/main.css
• Fixes the open <select> popup rendering as a light GTK panel on Linux even when our app chrome is dark
• Reported against v0.8.0-rc.3 on the language picker in Settings

Why

On Linux, Tauri renders via webkit2gtk. The closed <select> control is styled by our CSS (bg-autonomi-dark, etc.) but the open popup is painted by GTK, which uses the system theme — and most distros default to a light theme. Our CSS can't reach into the GTK popup; we need to tell the webview which variant of native controls to render. color-scheme is the W3C-standard property for exactly this:

Specifies which color schemes the element is comfortable being rendered in. The user agent then uses an appropriate variant of native controls.

Side effects (all positive):

• Scrollbars also pick up the dark variant on Linux
• Other native controls (date pickers, file inputs) follow the same rule

Why now

The language picker is the entry point for any future CJK font-banner testing on Linux — testers need to switch to ja to see the banner. A jarring light-on-dark dropdown popup right at that entry point reads as broken even though the rest of the UI is fine.

Test plan

• Linux: open language dropdown — popup is dark
• Linux: toggle to light mode — popup is light
• Windows / macOS: no visual regression (was already correct via OS)
• vue-tsc clean (no JS / TS changes)
• npm run build clean

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