fix: titlebar color

fix: markdown format issues

* fix: markdown format issues
* fix: center elements on the page

fix: blank page

fix: LaTeX format in exported markdown

* fix: LaTeX format in exported markdown
* fix: copy issue

fix: login issue

feat: support keyboard shortcuts

fix: network settings error

feat: support markdown and pdf export

fix: login issue

feat: customize appearance (theme & font size)

* feat: customize appearance (theme & font size)

* feat: save user preferences to config.json