Now that the view transition animation is gone, setTheme no longer
needs click coordinates. Reduce the API surface to (theme: Theme) =>
void and simplify the call sites in mode-toggle and ThemeSettings.
The View Transitions API used here crashes WebKitGTK with SIGSEGV on
Linux. Rather than gating document.startViewTransition per platform
(see PR #2502), drop the animation entirely — it's a low-value visual
flourish on a low-frequency action that doesn't justify a permanent
platform branch.
Removes the ::view-transition-* CSS block and the coordinate plumbing
in setTheme. The optional event parameter is kept on the API surface
to keep call sites compiling; they'll be cleaned up in a follow-up
commit.
Pass "system" to set_window_theme instead of explicitly detecting dark/light,
so Tauri uses window.set_theme(None) and the WebView's prefers-color-scheme
media query stays in sync with the real OS theme.
Jason