Editor opens in native windows (one per file)

Real BrowserWindow per edited file with its own RPC instance, deduped by
sessionId+path. The same Angular bundle boots an editor-only root via
preload globals. Native close isn't cancellable in electrobun, so the
editor mirrors its draft to bun; traffic-light closes get a native
Save/Discard prompt after the fact, and failed saves park the draft in
the edit-leftovers dir. In-window closes (Cmd+W, Close) use a styled
prompt; the quit guard now also counts unsaved editors.

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

ui/src/app/platform.ts

@@ -1,12 +1,25 @@
-// Injected by the bun process via BrowserWindow preload before any page script.
-// Brand-neutral name so rebrands never touch this file.
+// Injected by the bun process via BrowserWindow preload (src/bun/windows.ts
+// buildPreload) before any page script. Brand-neutral names so rebrands never
+// touch this file.
+import type { EditorOpenContext } from '@shared/rpc';
+
 declare global {
   interface Window {
     __APP_PLATFORM?: string;
+    /** Which root component this window boots: the full app or one editor. */
+    __APP_MODE?: 'main' | 'editor';
+    /** Window-kind specific context (EditorOpenContext for editor windows). */
+    __APP_CTX?: unknown;
   }
 }
 
 export const PLATFORM = typeof window !== 'undefined' ? (window.__APP_PLATFORM ?? '') : '';
 
-/** True when the window uses macOS hiddenInset chrome (tab bar fused with titlebar). */
+/** True when the window uses macOS hiddenInset chrome (header fused with titlebar). */
 export const IS_MAC_CHROME = PLATFORM === 'darwin';
+
+export const APP_MODE: 'main' | 'editor' =
+  typeof window !== 'undefined' ? (window.__APP_MODE ?? 'main') : 'main';
+
+export const EDITOR_CONTEXT =
+  typeof window !== 'undefined' ? (window.__APP_CTX as EditorOpenContext | undefined) : undefined;