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/platform-notes.md

@@ -17,7 +17,7 @@ macOS is the primary target; Windows 11+ x64 and Linux x64 are **beta** targets
 | Shell wording | `ui context-menu.service.ts` | Reveal in Finder / Move to Trash | Show in Explorer / Move to Recycle Bin | Show in File Manager |
 | Shortcut labels | `ui platform.ts MOD_LABEL/ALT_LABEL` | ⌘ / ⌥ | Ctrl+ / Alt+ | Ctrl+ / Alt+ |
 | Tray | `src/bun/tray.ts` | 22 pt template PNG | 16 px colored PNG | colored (needs AppIndicator, see below) |
-| App menu | `src/bun/menu.ts` | full menu incl. About | none (early return) — About lives in the options popover | none (unsupported upstream) |
+| App menu | `src/bun/menu.ts` | full menu incl. About | none (early return) — Settings opens from the tab-bar gear (Ctrl+,), About from its About section | none (unsupported upstream) |
 | connections.json protection | `store/connections.ts` | chmod 0600 | `icacls` current-user ACL (0600 is meaningless) | 0600 |
 | App data dir | `store/paths.ts` | ~/Library/Application Support | %APPDATA% | $XDG_CONFIG_HOME/.config |
 | ssh-agent | `fs/sftp.ts` | $SSH_AUTH_SOCK | `\\.\pipe\openssh-ssh-agent` fallback | $SSH_AUTH_SOCK |
@@ -172,7 +172,7 @@ Code paths written blind (no Windows box was available); verify in order:
 7. Ctrl+T/W/F/S and Ctrl+digit tab jumps; tooltips read "Ctrl+…".
 8. connections.json ACL: `icacls connections.json` shows only the current
    user after saving a connection.
-9. Editor/manager windows open, modal bounce works, About shows from the
-   options popover.
+9. Editor/manager/Settings windows open, modal bounce works, Ctrl+, opens
+   Settings and About shows from its About section.
 10. Known-issue confirmation: HiDPI blur on a scaled display (expected until
     upstream fixes it).