fix(preview): refuse a bare ~ folder in a Docker case too (#604 merge-time fix)

The Docker refusal on POST /api/sessions/:id/folders tested only `~/`, but
listFolder also expands a bare `~` to this host's home, so a Docker case
could list and serve the host home through it (the preview's "Show folder"
for a `~/x.png` link reached it with no typing). A bare `~` now gets the same
400.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/routes/file-routes.ts

@@ -2404,7 +2404,8 @@ export function registerFileRoutes(app: FastifyInstance, ctx: SessionPort & Even
       reply.code(400).send(createErrorResponse(ApiErrorCode.INVALID_INPUT, 'Missing folder path'));
       return;
     }
-    if (scope.docker && body.path.startsWith('~/')) {
+    // A bare `~` expands to this host's home too, not the container's.
+    if (scope.docker && (body.path === '~' || body.path.startsWith('~/'))) {
       reply.code(400).send(createErrorResponse(ApiErrorCode.INVALID_INPUT, DOCKER_HOME_PATH_ERROR));
       return;
     }

test/routes/folder-grid-routes.test.ts

@@ -341,6 +341,8 @@ describe('folder grid routes — local folder', () => {
       const home = await grant('~/renders');
       expect(home.statusCode).toBe(400);
       expect(JSON.parse(home.body).error).toMatch(/docker/i);
+      // A bare `~` is this host's home as much as `~/` is.
+      expect((await grant('~')).statusCode).toBe(400);
       expect((await grant(dir)).statusCode).toBe(200);
     });