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/types/settings.ts

@@ -1,5 +1,13 @@
 import type { ColumnWidth } from "./board";
 
+export interface WindowState {
+  x: number;
+  y: number;
+  width: number;
+  height: number;
+  maximized: boolean;
+}
+
 export interface AppSettings {
   theme: "light" | "dark" | "system";
   dataDirectory: string | null;
@@ -8,4 +16,5 @@ export interface AppSettings {
   uiZoom: number;
   density: "compact" | "comfortable" | "spacious";
   defaultColumnWidth: ColumnWidth;
+  windowState: WindowState | null;
 }