fix(windows,linux): reduce dark mode white flash on window open

Detect system dark mode on Windows (registry AppsUseLightTheme) and
Linux (gsettings for GNOME, kreadconfig for KDE Plasma) to set native
window background color before webview renders, matching the existing
macOS behavior. Make should_use_dark_background() cross-platform and
apply background_color in all platform branches of
apply_platform_window_style.

src-tauri/src/commands/windows.rs

@@ -96,7 +96,7 @@ fn is_system_dark_mode() -> bool {
     use std::sync::OnceLock;
     static CACHED: OnceLock<bool> = OnceLock::new();
     *CACHED.get_or_init(|| {
-        std::process::Command::new("defaults")
+        crate::process::std_command("defaults")
             .args(["read", "-g", "AppleInterfaceStyle"])
             .output()
             .map(|o| o.status.success()) // key exists only in dark mode
@@ -104,9 +104,86 @@ fn is_system_dark_mode() -> bool {
     })
 }
 
+/// Detect Windows system dark mode via registry query.
+/// `AppsUseLightTheme`: 0 = dark, 1 = light.
+/// Uses `crate::process::std_command` to avoid flashing a console window.
+/// On pre-1809 Windows where the key is absent, defaults to light mode.
+#[cfg(target_os = "windows")]
+fn is_system_dark_mode() -> bool {
+    use std::sync::OnceLock;
+    static CACHED: OnceLock<bool> = OnceLock::new();
+    *CACHED.get_or_init(|| {
+        crate::process::std_command("reg")
+            .args([
+                "query",
+                r"HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize",
+                "/v",
+                "AppsUseLightTheme",
+            ])
+            .output()
+            .ok()
+            .and_then(|o| {
+                let stdout = String::from_utf8_lossy(&o.stdout);
+                // Output: "    AppsUseLightTheme    REG_DWORD    0x0"
+                // Extract the last token on the matching line to avoid
+                // substring false-positives (e.g. "0x00000001" contains "0x0").
+                stdout.lines().find(|l| l.contains("AppsUseLightTheme")).map(|line| {
+                    line.split_whitespace()
+                        .last()
+                        .map(|val| val == "0x0" || val == "0x00000000")
+                        .unwrap_or(false)
+                })
+            })
+            .unwrap_or(false)
+    })
+}
+
+/// Detect Linux system dark mode via desktop environment settings.
+/// Covers GNOME (gsettings) and KDE Plasma (kreadconfig5/6).
+/// Falls back to light mode on unsupported desktops (XFCE, etc.).
+#[cfg(target_os = "linux")]
+fn is_system_dark_mode() -> bool {
+    use std::sync::OnceLock;
+    static CACHED: OnceLock<bool> = OnceLock::new();
+    *CACHED.get_or_init(|| {
+        // GNOME 42+: color-scheme = 'prefer-dark'
+        if let Ok(output) = crate::process::std_command("gsettings")
+            .args(["get", "org.gnome.desktop.interface", "color-scheme"])
+            .output()
+        {
+            let s = String::from_utf8_lossy(&output.stdout);
+            if s.contains("prefer-dark") {
+                return true;
+            }
+        }
+        // Older GNOME / GTK: theme name contains "dark"
+        if let Ok(output) = crate::process::std_command("gsettings")
+            .args(["get", "org.gnome.desktop.interface", "gtk-theme"])
+            .output()
+        {
+            let s = String::from_utf8_lossy(&output.stdout).to_lowercase();
+            if s.contains("dark") {
+                return true;
+            }
+        }
+        // KDE Plasma 5/6: ColorScheme name contains "dark"
+        for cmd in ["kreadconfig6", "kreadconfig5"] {
+            if let Ok(output) = crate::process::std_command(cmd)
+                .args(["--group", "General", "--key", "ColorScheme"])
+                .output()
+            {
+                let s = String::from_utf8_lossy(&output.stdout).to_lowercase();
+                if s.contains("dark") {
+                    return true;
+                }
+            }
+        }
+        false
+    })
+}
+
 /// Determine whether the window should use a dark background, considering
 /// both the user's explicit preference (from DB) and the OS appearance.
-#[cfg(target_os = "macos")]
 fn should_use_dark_background() -> bool {
     match CACHED_APPEARANCE_MODE.load(AtomicOrdering::Relaxed) {
         MODE_DARK => true,
@@ -138,12 +215,21 @@ where
 
     #[cfg(target_os = "windows")]
     {
+        let builder = if should_use_dark_background() {
+            builder.background_color(tauri::window::Color(9, 9, 11, 255))
+        } else {
+            builder
+        };
         return builder.decorations(false);
     }
 
     #[cfg(not(any(target_os = "macos", target_os = "windows")))]
     {
-        builder
+        if should_use_dark_background() {
+            builder.background_color(tauri::window::Color(9, 9, 11, 255))
+        } else {
+            builder
+        }
     }
 }