feat: change default divider thickness to 3px for first-time users (#256)

The previous 1px default was too thin for comfortable visibility.
This changes the default to 3px, which only affects new users who
haven't customized the setting. Also updates the preview component's
fallback to stay consistent with the new default.

src/renderer/src/components/settings/TerminalThemePreview.tsx

@@ -14,7 +14,7 @@ export function TerminalThemePreview({
   title,
   description,
   appearance,
-  dividerThicknessPx = 1,
+  dividerThicknessPx = 3,
   inactivePaneOpacity = 0.9,
   activePaneOpacity = 1
 }: TerminalThemePreviewProps): React.JSX.Element {

src/shared/constants.ts

@@ -50,7 +50,7 @@ export function getDefaultSettings(homedir: string): GlobalSettings {
     terminalInactivePaneOpacity: 0.8,
     terminalActivePaneOpacity: 1,
     terminalPaneOpacityTransitionMs: 140,
-    terminalDividerThicknessPx: 1,
+    terminalDividerThicknessPx: 3,
     terminalScrollbackBytes: 10_000_000,
     rightSidebarOpenByDefault: true
   }