Move settings into a standalone Settings window

The tab-bar popover had outgrown itself. Settings now open in their own
native window (a fourth window kind, built like the connection manager but
not modal): a sidebar with Appearance, Transfers, Editor, File list and
About, holding every popover row plus the visible-columns choice. Each
control is an ordinary settings save, so the other windows follow live
while it stays open.

The gear button in the tab bar, Ctrl+, and the macOS app menu's
"Settings..." (Cmd+,) open it. On macOS Cmd+W arrives as the main window's
close-tab menu action, so bun closes the focused Settings window first
instead of a main-window tab.

Co-Authored-By: Claude <noreply@anthropic.com>

src/bun/windows.ts

@@ -13,7 +13,7 @@ import type { AppSettings } from "../shared/types";
  * windows flash the wrong background.
  */
 export function buildPreload(
-	mode: "main" | "editor" | "manager",
+	mode: "main" | "editor" | "manager" | "settings",
 	settings: AppSettings,
 	ctx?: unknown,
 ): string {
@@ -76,7 +76,7 @@ export interface SecondaryWindowOptions {
 	title: string;
 	/** The once-resolved dev/prod url (see getMainViewUrl in index.ts). */
 	url: string;
-	mode: "editor" | "manager";
+	mode: "editor" | "manager" | "settings";
 	/** Current persisted settings — baked into the preload for flash-free paint. */
 	settings: AppSettings;
 	ctx?: unknown;