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/stores/app-store.ts

@@ -62,6 +62,7 @@ export const useAppStore = create<AppState>((set, get) => ({
     uiZoom: 1,
     density: "comfortable",
     defaultColumnWidth: "standard",
+    windowState: null,
   },
   boards: [],
   view: { type: "board-list" },