`--output-format=html` incorrectly encodes some characters

**Describe the bug** When using `--output-format=html` (e.g. in `kart show` or `kart diff` CLI), some characters will be encoded in a way that makes the resulting html page fail to load. **To Reproduce** Steps to reproduce the behaviour: 1. Modify a dataset so a field includes a special character such as `/`,`<`,`>`. 2. `kart show --output-format=html --output show.html` 3. A browser will open, displaying show.html, but no page content. 4. Open browser dev tools --> console. 5. See error (similar to "Uncaught SyntaxError: JSON.parse: bad escaped character at line 1 column 85159 of the JSON data") 6. Page source will include something like `{"fid": 1, "name": "a name \x3c\x2f\x3e with special chars"}` **Expected behaviour** What did you expect to happen? The HTML page should work, displaying a map and table of changes the same way as when there are no special characters. **Output** Add the output you're seeing to help explain your problem. **Version Info ** - OS: `Ubuntu 24.04.3 LTS (6.11.0-29-generic x86_64 GNU/Linux)` - Version: [from `kart version`] ``` Kart v0.16.1, Copyright (c) Kart Contributors » GDAL v3.8.5; PROJ v9.4.0; PDAL v2.7.2 » PyGit2 v1.12.1; Libgit2 v1.6.4; Git v2.47.1; Git LFS v3.3.0 » SQLAlchemy v1.4.45; pysqlite3 v2.6.0/v3.45.3; SpatiaLite v5.1.0; Libpq v16.0.2 ```