fix(picker): do not offer New folder inside a cases directory

A folder made in a cases directory is already a case (no scaffold), which Link
Existing then refuses by name and which deleting would take the linked folder
with. The route refuses a cases directory as the parent (the shared one and the
caller's own), a leading tilde in the name, and maps a vanished parent to 404.
Docs now say every name it accepts is one Link Existing accepts, and the wiki
describes the button.

src/web/routes/file-routes.ts

@@ -57,6 +57,7 @@ import {
   findSessionOrFail,
   getAuthUser,
   parseBody,
+  resolveCasesDir,
   validateSessionFilePath,
   validateSessionFilePathLexical,
 } from '../route-helpers.js';
@@ -1468,6 +1469,21 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
       reply.code(403);
       return createErrorResponse(ApiErrorCode.INVALID_INPUT, 'Folders cannot be created here');
     }
+    // The cases directories (shared, and the caller's own in multi-user mode) are what GET /api/cases lists as
+    // local cases: a folder made there is a case with no scaffold, which Link then refuses by name and which
+    // deleting would take the linked folder with. A case there is made through Add Case > New Case.
+    const casesDirs = await Promise.all(
+      [...new Set([CASES_DIR, resolveCasesDir(getAuthUser(req))])].map((dir) =>
+        fs.realpath(dir).catch(() => resolve(dir))
+      )
+    );
+    if (casesDirs.includes(resolvedPath)) {
+      reply.code(400);
+      return createErrorResponse(
+        ApiErrorCode.INVALID_INPUT,
+        'This is a cases folder: every folder in it is a case. Use Add Case > New Case to make one there.'
+      );
+    }
     // Like browse, a file is not a place to create in.
     try {
       if (!(await fs.stat(resolvedPath)).isDirectory()) throw new Error('not a directory');
@@ -1488,6 +1504,10 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
         reply.code(403);
         return createErrorResponse(ApiErrorCode.INVALID_INPUT, 'This folder is not writable');
       }
+      if (code === 'ENOENT') {
+        reply.code(404);
+        return createErrorResponse(ApiErrorCode.NOT_FOUND, `Folder not found: ${candidatePath}`);
+      }
       if (code === 'ENAMETOOLONG') {
         reply.code(400);
         return createErrorResponse(ApiErrorCode.INVALID_INPUT, 'Folder name is too long');

src/web/schemas.ts

@@ -85,8 +85,8 @@ const showHiddenQuerySchema = z.enum(['true', 'false']).optional();
 /**
  * Body for the path picker's "New folder": one folder name created directly inside `path`. The name is held
  * to the character set `safePathSchema` accepts (letters, digits, `_ - . ~` and space, no `..`), so every
- * folder this makes is one Link Existing and a session working directory will accept, and never a control
- * character. It is also one path segment and not dot-prefixed (the picker hides those by default), and at
+ * name it accepts is one Link Existing and a session working directory accept (the parent's own path is
+ * judged there separately), and never a control character. It is also one path segment, not dot-prefixed (the picker hides those by default) and not tilde-prefixed (a folder named `~` is an `rm -rf ~` accident), and at
  * most 255 BYTES, the usual filesystem limit.
  */
 export const FilesystemMkdirSchema = z.object({
@@ -99,8 +99,8 @@ export const FilesystemMkdirSchema = z.object({
     .refine((n) => /^[\p{L}\p{N}_\-. ~]+$/u.test(n), {
       message: 'Folder name can only use letters, numbers, spaces and _ - . ~',
     })
-    .refine((n) => !n.includes('..') && !n.startsWith('.'), {
-      message: 'Folder name cannot contain ".." or start with a dot',
+    .refine((n) => !n.includes('..') && !n.startsWith('.') && !n.startsWith('~'), {
+      message: 'Folder name cannot contain "..", or start with a dot or a tilde',
     }),
   sessionId: z
     .string()