Connection manager in its own native window, modal to main

Single window centered over the main app. Electrobun has no native
modality, so it's approximated: bun bounces the main window's focus back
to the manager, the main webview shows a click-swallowing overlay, and
main-targeted menu/keyboard actions are dropped while open. Editor
windows stay fully usable. Connect/open-stack execute in the main
window via the existing connect: action channel; store edits push a
connectionsChanged refresh. Trigger moved from the panel dropdown to a
tab-bar button.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/file-panel/file-panel.ts

@@ -121,12 +121,6 @@ export class FilePanel {
     return `${host.username}@${host.hostname}:${host.port}`;
   }
 
-  openManager(): void {
-    this.menuOpen.set(false);
-    this.activate();
-    this.dialogs.manager.set(true);
-  }
-
   iconFor(entry: FileEntry): string {
     return fileGlyph(entry, this.settings.iconColors() === 'multi');
   }