fix: use hyphenated permission identifiers for Tauri v2 ACL

Tauri v2 ACL identifiers only allow lowercase ASCII letters,
hyphens, and optional prefix with colon. Changed:
- fs_exist_dir -> fs-exist-dir
- fs_make_dir -> fs-make-dir

Author: VirtualHotBar

src-tauri/capabilities/migrated.json

@@ -195,7 +195,7 @@
     "dialog:default",
     "dialog:allow-open",
     "dialog:allow-save",
-    "fs_exist_dir",
-    "fs_make_dir"
+    "fs-exist-dir",
+    "fs-make-dir"
   ]
 }