* fix: restore T2I text template rendering
- keep using {{ text | safe }} instead of text_base64
- inject Shiki runtime by default for T2I templates
- update built-in templates to read markdown from a hidden textarea
- improve WebUI preview sample text and Shiki runtime serving
- add regression tests for template rendering and runtime injection
* fix: prevent injected Shiki runtime from breaking T2I templates
* fix(t2i): restore raw text template rendering
* test(t2i): remove test
* fix(t2i): restore previewText