feat: narrow-window behavior — sidebar auto-close, chrome wrap, 640px floor

The sidebar no longer squeezes the page when the window narrows; the
page itself never goes below a comfortable floor (owner decisions,
2026-09-17: 640px min, widen-window on toggle, auto-restore):

- minWidth 640 / minHeight 600 on the BrowserWindow (macOS enforces)
- Below SIDEBAR_AUTOCLOSE (928 = 640 + 288 sidebar) the sidebar
  auto-closes, and auto-RESTORES when the window widens past it again —
  both fall out of one expression: rendered = pref && !narrow, where
  pref is the persisted user choice. Manual closes persist and stay
  closed; auto-close never touches the preference.
- Opening the sidebar while narrow asks the shell to widen the window
  to 928 first (new lh:expand-window IPC, preload.cjs contextBridge —
  the renderer's first and only Electron bridge): the toggle is never a
  dead button, and the layout never has to survive narrow + sidebar.
  In the plain browser there is no suppression and no widening — a
  narrow tab opens the sidebar if the user says so.
- Graph chrome row wraps: max-w-[calc(100%-9rem)] + flex-wrap keeps the
  row clear of the header toggle cluster and stacks it onto extra lines
  below ~640px instead of overflowing left.

State machine verified in the Electron dev shell across all six
transitions (auto-close, auto-restore, manual-close-persists,
reopen-widens, browser fallback, wrap at 640/500/360).

electron/main.cjs

@@ -21,13 +21,17 @@
  *     settings milestone builds the UI.
  */
 
-const { app, BrowserWindow, Menu, session, shell } = require("electron");
+const { app, BrowserWindow, Menu, ipcMain, session, shell } = require("electron");
 const { spawn } = require("node:child_process");
 const fs = require("node:fs");
 const http = require("node:http");
 const net = require("node:net");
 const path = require("node:path");
 
+// Mirrors SIDEBAR_AUTOCLOSE in components/sidebar.tsx — keep in sync (the
+// renderer owns the number; this is the IPC widen target).
+const SIDEBAR_AUTOCLOSE = 928;
+
 const PRODUCT_NAME = "LearnHarness";
 const AUTH_FILE = () => path.join(app.getPath("userData"), "auth.json");
 const DATA_DIR = () => path.join(app.getPath("userData"), "data");
@@ -199,6 +203,7 @@ function createWindow(url) {
 			contextIsolation: true,
 			sandbox: true,
 			nodeIntegration: false,
+			preload: path.join(__dirname, "preload.cjs"),
 		},
 		// Live-resize + first-paint flash (owner, 2026-09-17): during a resize
 		// the native window surface changes size before Chromium's compositor
@@ -209,6 +214,14 @@ function createWindow(url) {
 		// appears only once the page has actually rendered.
 		backgroundColor: "#141414",
 		show: false,
+		// Narrow-window behavior (owner, 2026-09-17): 640px is the floor —
+		// below it even a wrapped chrome row gets cramped and the chat header
+		// (lights gutter 92 + toggle + title) runs out of room. The sidebar
+		// auto-closes below 928 = 640 + 288 (sidebar) in the renderer
+		// (components/sidebar.tsx, SIDEBAR_AUTOCLOSE); its "lh:expand-window"
+		// IPC widens the window back to that threshold.
+		minWidth: 640,
+		minHeight: 600,
 	});
 
 	const allowedOrigin = () => new URL(url).origin;
@@ -293,6 +306,19 @@ async function main() {
 	}
 
 	installMenu();
+
+	// Sidebar-at-narrow-window (owner, 2026-09-17): the renderer auto-closes
+	// the sidebar below SIDEBAR_AUTOCLOSE (928px); opening it again at that
+	// width asks the window to widen instead — the layout never has to
+	// accommodate a narrow window + open sidebar. Top-left stays fixed, the
+	// window grows right. No-op when already wide.
+	ipcMain.on("lh:expand-window", (event) => {
+		const win = BrowserWindow.fromWebContents(event.sender);
+		if (!win) return;
+		const { x, y, width, height } = win.getBounds();
+		const target = SIDEBAR_AUTOCLOSE;
+		if (width < target) win.setBounds({ x, y, width: target, height });
+	});
 	app.on("activate", () => {
 		if (BrowserWindow.getAllWindows().length === 0) createWindow(url);
 	});