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/themes/theme-gitea-dark.css

@@ -1,6 +1,3 @@
-@import "../chroma/dark.css";
-@import "../codemirror/dark.css";
-
 gitea-theme-meta-info {
   --theme-display-name: "Dark";
   --theme-color-scheme: "dark";
@@ -224,7 +221,8 @@ gitea-theme-meta-info {
   --color-secondary-bg: #2a3137;
   --color-expand-button: #2f363d;
   --color-placeholder-text: var(--color-text-light-3);
-  --color-editor-line-highlight: var(--color-primary-light-5);
+  --color-editor-line-highlight: var(--color-secondary-alpha-40);
+  --color-editor-selection: var(--color-primary-alpha-50);
   --color-project-column-bg: var(--color-secondary-light-2);
   --color-caret: var(--color-text); /* should ideally be --color-text-dark, see #15651 */
   --color-reaction-bg: #e8f3ff12;
@@ -243,12 +241,53 @@ gitea-theme-meta-info {
   --color-accent: var(--color-primary-light-1);
   --color-small-accent: var(--color-primary-light-5);
   --color-highlight-fg: #87651e;
-  --color-highlight-bg: #352c1c;
+  --color-highlight-bg: #443a27;
   --color-overlay-backdrop: #080808c0;
   --color-danger: var(--color-red);
   --color-transparency-grid-light: #2a2a2a;
   --color-transparency-grid-dark: #1a1a1a;
   --color-workflow-edge-hover: #616e78;
+  --color-syntax-keyword: #ff8854;
+  --color-syntax-bool: #25bbc9;
+  --color-syntax-control: #dd9e17;
+  --color-syntax-name: #c7a618;
+  --color-syntax-type: #eb8cb3;
+  --color-syntax-number: #63b2dd;
+  --color-syntax-operator: #ff8854;
+  --color-syntax-regexp: #b89de4;
+  --color-syntax-string: #95b62a;
+  --color-syntax-comment: #8898b0;
+  --color-syntax-invalid: #ff8686;
+  --color-syntax-link: var(--color-primary);
+  --color-syntax-tag: #ff8854;
+  --color-syntax-attribute: #c792ff;
+  --color-syntax-property: #55afff;
+  --color-syntax-variable: #e29b33;
+  --color-syntax-string-special: #dd9e17;
+  --color-syntax-escape: #c7a618;
+  --color-syntax-entity: #c792ff;
+  --color-syntax-preproc: #4cbe7a;
+  --color-syntax-preproc-file: #63b2dd;
+  --color-syntax-decorator: #4cbe7a;
+  --color-syntax-namespace: #c9d1d9;
+  --color-syntax-name-pseudo: #c792ff;
+  --color-syntax-comment-special: #b89de4;
+  --color-syntax-text: #c9d1d9;
+  --color-syntax-text-alt: #b9bcc7;
+  --color-syntax-punctuation: #d2d4db;
+  --color-syntax-whitespace: #7f8699;
+  --color-syntax-diff-fg: #ffffff;
+  --color-syntax-deleted-bg: #5f3737;
+  --color-syntax-inserted-bg: #3a523a;
+  --color-syntax-emph: #d1a242;
+  --color-syntax-strong: #e29b33;
+  --color-syntax-heading: #dd9e17;
+  --color-syntax-subheading: #95b62a;
+  --color-syntax-output: #8898b0;
+  --color-syntax-prompt: #e29b33;
+  --color-syntax-traceback: #ff8686;
+  --color-syntax-matching-bracket-bg: #00918a48;
+  --color-syntax-nonmatching-bracket-bg: #cc484848;
   accent-color: var(--color-accent);
   color-scheme: dark;
 }