Inline and lazy-load EasyMDE CSS, fix border colors (#36714)

Replace the external easymde.min.css import with an inlined and
lazy-loaded CSS file that uses proper theme variables for border colors.
All EasyMDE/CodeMirror rules are scoped under `.EasyMDEContainer`,
removing the need for !important overrides.

- Fixes easymde borders, these were broken since a while now
- Scope all easymde styles to .EasyMDEContainer
- Inline easymde.min.css and codemirror.css into web_src/css/easymde.css
- Lazy-load the CSS alongside the JS in switchToEasyMDE()
- Fix .editor-toolbar and .CodeMirror border colors to use
--color-input-border matching textarea inputs
- Remove unused gutter, line number, and other unconfigured styles
- Move .editor-loading to codeeditor.css where it belongs

<img width="891" height="518" alt="image"
src="https://github.com/user-attachments/assets/87495de5-7872-4645-90e7-96fe0f782f02"
/>

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

web_src/css/features/codeeditor.css

@@ -1,3 +1,8 @@
+.editor-loading {
+  padding: 1rem;
+  text-align: center;
+}
+
 .monaco-editor-container,
 .editor-loading.is-loading {
   width: 100%;