Implement custom Windows title bar and theme refactor

Adds a frameless window with custom title bar controls for Windows, including minimize, maximize/restore, and close buttons. Refactors theme color application to use CSS variables and color-mix for improved consistency and dynamic theming across home, settings, and main UI. Updates renderer and styles to support the new title bar, platform detection, and improved color handling. No changes to lock files.

main.js

@@ -539,9 +539,11 @@ function createWindow(startUrl, bigPictureMode = false) {
       transparent: true,
     });
   } else if (process.platform === 'win32') {
+    // Use frameless window on Windows with custom title bar controls
+    // rendered in the tab strip area (Firefox-style).
     Object.assign(windowOptions, {
-      frame: true, // Use default Windows title bar.
-      // removed titleBarOverlay to restore native Windows controls.
+      frame: false,
+      backgroundColor: '#0b0d10',
     });
   } else {
     windowOptions.frame = true;
@@ -932,6 +934,9 @@ ipcMain.handle('window-maximize', event => {
 ipcMain.handle('window-close', event => {
   BrowserWindow.fromWebContents(event.sender).close();
 });
+ipcMain.handle('window-is-maximized', event => {
+  return BrowserWindow.fromWebContents(event.sender).isMaximized();
+});
 
 // Add site and search history IPC handlers
 // Site history is now handled via localStorage in the renderer