fix(cases): only refuse the cases folder and its direct children for a custom-folder case

blockedReason() refused every path below a cases directory, with the reason that it is
"already listed as a local case". GET /api/cases lists only the direct children of the cases
directory, so only those can be registered twice. With CODEMAN_CASES_PATH set to a broad root
such as /mnt/user/Scripts, the rule blocked almost the whole tree, including folders that are
already linked cases. Refuse the cases directory itself and its direct children only, and name
the cases directory in the message.

src/web/case-path.ts

@@ -98,9 +98,13 @@ export function blockedReason(
     const firstSegment = absPath.slice(ctx.home.length + 1).split(sep)[0];
     if (/^\.codeman/.test(firstSegment)) return "Codeman's own data folder cannot be a case";
   }
+  // Only the cases folder itself and its DIRECT children are listed as local cases (GET /api/cases reads
+  // that one level), so only those would be registered twice. A deeper folder is not listed and is a
+  // perfectly good linked case: with CODEMAN_CASES_PATH pointing at a broad root such as /mnt/user/Scripts,
+  // refusing everything below it blocked most of the tree.
   for (const dir of ctx.casesDirs ?? []) {
-    if (isWithin(absPath, dir)) {
-      return 'That folder is inside the cases folder; create a case there with plain Create New (no custom folder)';
+    if (absPath === dir || dirname(absPath) === dir) {
+      return `That folder is in the cases folder (${dir}), where it is already listed as a case; create it there with plain Create New (no custom folder)`;
     }
   }
   return null;