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-tauri/capabilities/default.json

@@ -17,8 +17,9 @@
     "dialog:default",
     "shell:default",
     "fs:default",
-    "fs:allow-appdata-read-recursive",
-    "fs:allow-appdata-write-recursive",
-    "fs:allow-appdata-meta-recursive"
+    "core:window:allow-set-size",
+    "core:window:allow-set-position",
+    "core:window:allow-outer-size",
+    "core:window:allow-outer-position"
   ]
 }