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/build-and-release.md

@@ -10,7 +10,7 @@ electrobun}`). It is regenerated by `postinstall` and at the start of every
 records, hence gitignored. Fresh checkouts fail `tsc` until `bun install` has
 run once (postinstall creates the file). `BRANDING.version` stays
 semver-shaped (`0.1.<n>`) for electrobun's config/updater; `BRANDING.buildLabel`
-("Build n") is what humans see (About box, options popover).
+("Build n") is what humans see (About box, Settings → About).
 
 ## Dev loop