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>

src/bun/windows.ts

@@ -7,7 +7,7 @@ const isMac = process.platform === "darwin";
  * reads them in ui/src/app/platform.ts before first paint (window chrome and
  * root-component selection both depend on them).
  */
-export function buildPreload(mode: "main" | "editor", ctx?: unknown): string {
+export function buildPreload(mode: "main" | "editor" | "manager", ctx?: unknown): string {
 	let script =
 		`window.__APP_PLATFORM=${JSON.stringify(process.platform)};` +
 		`window.__APP_MODE=${JSON.stringify(mode)};`;
@@ -19,7 +19,7 @@ export interface SecondaryWindowOptions {
 	title: string;
 	/** The once-resolved dev/prod url (see getMainViewUrl in index.ts). */
 	url: string;
-	mode: "editor"; // widens when more window kinds exist (connection manager…)
+	mode: "editor" | "manager";
 	ctx?: unknown;
 	frame: { x: number; y: number; width: number; height: number };
 	/** Per-window rpc instance. NEVER share one across windows — each