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>

README.md

@@ -36,7 +36,7 @@ Dense reference material lives in [docs/](docs/): [architecture](docs/architectu
 - **Menu-bar (tray) icon**: template icon that adapts to light/dark; click it
   to bring the app to the front.
 - **Edit files** (F4 / toolbar / File menu): a built-in editor works on local
-  and remote files directly; or pick an external app (Options → Editor →
+  and remote files directly; or pick an external app (Settings → Editor →
   Custom → Choose…). External edits of remote files download to a temp folder,
   are watched, and every save auto-uploads (visible in the transfer list) until
   the session disconnects. Leftover temp files from a crash are offered for
@@ -181,7 +181,7 @@ Saved connections live in a plain JSON file in the OS app-data directory
 `Space` select · `Enter` open folder · `Backspace` parent ·
 `Ctrl/Cmd+A` select all · `F5` send to other panel · `F2` rename ·
 `F7` new folder · `F8`/`Delete` delete · `Cmd/Ctrl+T` new tab ·
-`Cmd/Ctrl+W` close tab · `Ctrl+Tab` next tab ·
+`Cmd/Ctrl+W` close tab · `Ctrl+Tab` next tab · `Cmd/Ctrl+,` settings ·
 `Cmd/Ctrl+=`/`-`/`0` interface bigger / smaller / 100%.
 
 (On Macs, function keys may need `fn` — every action also has a toolbar button.)