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>

docs/gotchas.md

@@ -114,6 +114,11 @@ touching the adjacent code.
   app-level `before-quit` supports `event.response = {allow:false}`). All
   unsaved-state guards must either intercept app-controlled close paths
   (⌘W, buttons) or survive the close (editor draft mirroring).
+- **⌘W in any secondary window arrives as the MAIN window's `close-tab`
+  menu action** (macOS: the app menu intercepts it before the webview sees
+  the key). bun must check the focused secondary window first — the Settings
+  window, then editors — and close THAT; a new window kind that skips this
+  check makes ⌘W close a main-window tab behind it.
 - **One rpc instance per window** — BrowserView binding steals the rpc's
   transport; a shared instance silently sends to the wrong webview.
 - **RPC before socket open is silently lost.** Everything gates on the