Save window layout on close with modal prompt

[kordless]

Goal

When the user closes a Hyperia window, prompt them to save the current layout. On Yes, persist the layout under a stable per-window key in electron-store. On No, clear any previously saved layout for that window. On Cancel, abort the close.

Scope

• Intercept the close event on each BrowserWindow (currently app/ui/window.ts:521 clean())
• Show a dialog.showMessageBox with three buttons: Save, Don't Save, Cancel
• On Save: serialize the window's full layout state (see below) to electron-store under windowLayouts[windowUid]
• On Don't Save: delete windowLayouts[windowUid]
• On Cancel: event.preventDefault() and abort close

Window identity

Each window needs a UID stable across launches. Approach:

• On window creation (app/ui/window.ts:67), generate a UUID and attach it to the window object
• On restore from saved layout, reuse the original UUID from the saved entry
• Use this UID as the key in windowLayouts

What gets serialized

type SavedLayout = {
  windowUid: string;
  position: [number, number];
  size: [number, number];
  termGroups: ITermGroup[];           // full BSP tree from Redux
  activeSessionUid: string;
  cwds: Record<string, string>;       // sessionUid → cwd
  webUrls: Record<string, string>;    // sessionUid → URL (see #N web panes)
  stickyIds: string[];                // see #N sticky binding
  savedAt: number;
};

Implementation notes

• Layout state (termGroups) lives in the renderer's Redux store — needs an IPC roundtrip on close to dump state from renderer to main before the close completes
• Add a new RPC: 'get layout snapshot' that the main process sends to the renderer, which replies with the serialized termGroups + active session
• bridge.ts already tracks BSP geometry per session — but full split tree shape lives in renderer

Files touched

• app/ui/window.ts — close handler, modal, IPC roundtrip
• app/config/windows.ts — extend with windowLayouts accessor
• typings/hyper.d.ts — add SavedLayout type
• lib/index.tsx or renderer entry — handle 'get layout snapshot' RPC

Out of scope

Restore is #N (separate issue). Sticky binding and web pane URLs are #N and #N.