chore: use consistent relative paths for resources (#2683)

- always store resources with a relative path with forward slashes, which will be transformed as needed when the file is accessed

- fix an issue with thumbnail generation on Windows

- add several validations for local storage setting

- improve front-end error feedback when changing local storage

- add migrations to make existing resource paths relative (not needed, but improves database consistency)

store/db/postgres/migration/prod/0.19/00_resource.sql

@@ -0,0 +1,19 @@
+-- Make resource internal_path relative (to MEMOS_DATA) and replace backslash with slash
+-- This is a best-effort approach, but even if it fails, it won't break assets from loading
+UPDATE resource
+SET
+  internal_path = REPLACE (internal_path, '\', '/')
+WHERE
+  internal_path LIKE '%assets\\%';
+
+UPDATE resource
+SET
+  internal_path = REPLACE (
+    internal_path,
+    SUBSTRING(
+      internal_path
+      FROM
+        1 FOR POSITION('/assets' IN internal_path)
+    ),
+    ''
+  );