feat: custom scrollbars, portable storage, window state persistence

- Custom scrollbar CSS using ::-webkit-scrollbar for Tauri's Chromium WebView
- Portable storage: all data written next to exe in data/ folder instead of AppData
- Rust get_portable_data_dir command with runtime FS scope for exe directory
- Window size/position/maximized saved to settings on close, restored on startup

src/App.tsx

@@ -1,8 +1,10 @@
 import { useState, useEffect, useCallback } from "react";
+import { getCurrentWindow, LogicalSize, LogicalPosition } from "@tauri-apps/api/window";
 import { AnimatePresence, motion } from "framer-motion";
 import { springs, fadeSlideLeft, fadeSlideRight } from "@/lib/motion";
 import { useAppStore } from "@/stores/app-store";
 import { useBoardStore } from "@/stores/board-store";
+import { saveSettings } from "@/lib/storage";
 import { AppShell } from "@/components/layout/AppShell";
 import { BoardList } from "@/components/boards/BoardList";
 import { BoardView } from "@/components/board/BoardView";
@@ -21,13 +23,46 @@ export default function App() {
   const [shortcutHelpOpen, setShortcutHelpOpen] = useState(false);
 
   useEffect(() => {
-    init();
+    init().then(() => {
+      // Restore window state after settings are loaded
+      const { settings } = useAppStore.getState();
+      const ws = settings.windowState;
+      if (ws) {
+        const appWindow = getCurrentWindow();
+        if (ws.maximized) {
+          appWindow.maximize();
+        } else {
+          appWindow.setSize(new LogicalSize(ws.width, ws.height));
+          appWindow.setPosition(new LogicalPosition(ws.x, ws.y));
+        }
+      }
+    });
   }, [init]);
 
-  // Flush pending board saves before the app window closes
+  // Save window state + flush board saves before the app window closes
   useEffect(() => {
     function handleBeforeUnload() {
       useBoardStore.getState().closeBoard();
+
+      // Save window state synchronously-ish (fire and forget)
+      const appWindow = getCurrentWindow();
+      Promise.all([
+        appWindow.outerSize(),
+        appWindow.outerPosition(),
+        appWindow.isMaximized(),
+      ]).then(([size, position, maximized]) => {
+        const settings = useAppStore.getState().settings;
+        saveSettings({
+          ...settings,
+          windowState: {
+            x: position.x,
+            y: position.y,
+            width: size.width,
+            height: size.height,
+            maximized,
+          },
+        });
+      });
     }
     window.addEventListener("beforeunload", handleBeforeUnload);
     return () => {