Use CodeMirror editor

CLAUDE.md

@@ -16,12 +16,30 @@ go build .
 make deploy
 ```
 
+### Editor bundle (the one build-pipeline exception)
+
+The page editor uses CodeMirror 6, vendored as a single pre-built IIFE at
+`assets/editor/vendor/codemirror.bundle.js` and embedded via `embed.FS`. This is
+the **only** deliberate exception to the "no build pipeline" rule below — it is a
+one-time, committed artifact, not a runtime build. `go build` / `make deploy`
+never touch Node and only consume the committed bundle.
+
+Regenerate the bundle **only** when upgrading the `@codemirror/*` versions:
+
+```bash
+# bump versions in editor-build/package.json first, then:
+make editor          # runs `npm ci && npm run build` in editor-build/, rewrites the vendored bundle
+```
+
+Commit the regenerated `codemirror.bundle.js` and the updated
+`editor-build/package-lock.json`. `editor-build/node_modules/` is gitignored.
+
 ## HTTP API Surface
 
 | Method | Path | Behaviour |
 |--------|------|-----------|
 | GET | `/{path}/` | If folder exists: render `index.md` + list contents. If not: show empty create prompt. |
-| GET | `/{path}/?edit` | Mobile-friendly editor with `index.md` content in a textarea |
+| GET | `/{path}/?edit` | CodeMirror 6 editor initialized with `index.md` content |
 | POST | `/{path}` | Write `index.md` to disk; creates the folder if it does not exist yet |
 
 Non-existent paths without a trailing slash redirect to the slash form (GET only — POSTs
@@ -49,7 +67,7 @@ Prefer separate, human-readable `.html` files over inlined HTML strings in Go. E
 
 ## Frontend Rules
 
-- Vanilla JS only — no frameworks, no build pipeline
+- Vanilla JS only — no frameworks, no build pipeline (the single exception is the vendored CodeMirror editor bundle; see Build & Deploy)
 - Each feature gets its own JS file; global behaviour goes in `global-shortcuts.js`
 - Do not inline JS in templates or merge unrelated features into one file
 - `ALT+SHIFT` is the modifier for all keyboard shortcuts — do not introduce others