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 <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: Copilot <[email protected]>

web_src/css/index.css

@@ -52,7 +52,6 @@
 @import "./markup/asciicast.css";
 
 @import "./chroma/base.css";
-@import "./codemirror/base.css";
 @import "./font_i18n.css";
 @import "./base.css";
 @import "./home.css";
@@ -74,7 +73,6 @@
 @import "./repo/commit-sign.css";
 @import "./repo/packages.css";
 
-@import "./editor/fileeditor.css";
 @import "./editor/combomarkdowneditor.css";
 
 @import "./org.css";