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/editor/fileeditor.css

@@ -1,56 +0,0 @@
-.editor-toolbar {
-  border-color: var(--color-secondary);
-}
-
-.editor-toolbar.fullscreen {
-  background: var(--color-body);
-}
-
-.editor-toolbar button {
-  border: none !important;
-  color: var(--color-text-light);
-}
-
-.editor-toolbar button:not(:hover) {
-  background-color: transparent !important;
-}
-
-.editor-toolbar i.separator {
-  border-left: none;
-  border-right-color: var(--color-secondary);
-}
-
-.editor-toolbar button:hover {
-  background: var(--color-hover);
-}
-
-.editor-toolbar button.active {
-  background: var(--color-active);
-}
-
-/* hide preview button, we have the preview tab for this */
-.editor-toolbar:not(.fullscreen) .preview {
-  display: none;
-}
-
-/* hide revert button in fullscreen, it breaks the page */
-.editor-toolbar.fullscreen .revert-to-textarea {
-  display: none;
-}
-
-.editor-preview {
-  background-color: var(--color-body);
-}
-
-.editor-preview-side {
-  border-color: var(--color-secondary);
-}
-
-.editor-statusbar {
-  color: var(--color-text-light);
-}
-
-.editor-loading {
-  padding: 1rem;
-  text-align: center;
-}