Replace Monaco with CodeMirror (#36764)

- Replace monaco-editor with CodeMirror 6
- Add `--color-syntax-*` CSS variables for all syntax token types,
shared by CodeMirror, Chroma and EasyMDE
- Consolidate chroma CSS into a single theme-independent file
(`modules/chroma.css`)
- Syntax colors in the code editor now match the code view and
light/dark themes
- Code editor is now 12px instead of 14px font size to match code view
and GitHub
- Use a global style for kbd elements
- When editing existing files, focus will be on codemirror instead of
filename input.
- Keyboard shortcuts are roughtly the same as VSCode
- Add a "Find" button, useful for mobile
- Add context menu similar to Monaco
- Add a command palette (Ctrl/Cmd+Shift+P or F1) or via button
- Add clickable URLs via Ctrl/Cmd+click
- Add e2e test for the code editor
- Remove `window.codeEditors` global
- The main missing Monaco features are hover types and semantic rename
but these were not fully working because monaco operated only on single
files and only for JS/TS/HTML/CSS/JSON.

| | Monaco (main) | CodeMirror (cm) | Delta |
|---|---|---|---|
| **Build time** | 7.8s | 5.3s | **-32%** |
| **JS output** | 25 MB | 14 MB | **-44%** |
| **CSS output** | 1.2 MB | 1012 KB | **-17%** |
| **Total (no maps)** | 23.3 MB | 12.1 MB | **-48%** |

Fixes: #36311
Fixes: #14776
Fixes: #12171

<img width="1333" height="555" alt="image"
src="https://github.com/user-attachments/assets/f0fe3a28-1ed9-4f22-bf25-2b161501d7ce"
/>

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/css/repo.css

@@ -207,6 +207,10 @@ td .commit-summary {
   padding: 0 !important;
 }
 
+.repo-editor-menu {
+  min-height: auto !important;
+}
+
 .repo-editor-header {
   display: flex;
   margin: 1rem 0;
@@ -841,7 +845,7 @@ td .commit-summary {
   width: 1%;
   min-width: 50px;
   font-family: monospace;
-  line-height: 20px;
+  line-height: var(--line-height-code);
   color: var(--color-text-light-1);
   white-space: nowrap;
   vertical-align: top;
@@ -1763,13 +1767,23 @@ tbody.commit-list {
 }
 
 .removed-code {
+  color: var(--color-text);
   background: var(--color-diff-removed-word-bg);
 }
 
+.removed-code span {
+  color: inherit !important;
+}
+
 .added-code {
+  color: var(--color-text);
   background: var(--color-diff-added-word-bg);
 }
 
+.added-code span {
+  color: inherit !important;
+}
+
 .code-diff-unified .del-code,
 .code-diff-unified .del-code td,
 .code-diff-split .del-code .lines-num-old,