Import the exercise builder

The drag-and-drop workspace for building BI122G exercise sheets, served
by the Workshop at /projects/exercise-builder. Plain HTML, CSS and ES
modules; no build step. It runs entirely in the browser — work in
progress lives in localStorage and finished sheets leave as downloads,
so nothing is written on the server.

This directory is canonical. It began as a deployment target for
deploy_builder.sh syncing from the BI122G course folder, and that
arrangement has ended; see the README.

Includes the sheet's light/dark switch: the stylesheet already carried
both themes and followed the reader's system setting, so this adds the
control that overrides it — a corner toggle built by sheet.js, applied
before first paint by a head script the exporter writes, and hidden in
print. The builder's toolbar previews the same attribute on the canvas.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ed5kU3HfZq9dfj7EYj5J2v

README.md

@@ -0,0 +1,59 @@
+# Exercise Builder — hosted copy
+
+The drag-and-drop workspace for building BI122G exercise sheets, served by the
+Workshop at:
+
+    /projects/exercise-builder          landing page
+    /projects/exercise-builder/app      the builder itself
+
+Open to everyone; no sign-in.
+
+## What is here
+
+    app/        the builder — plain HTML, CSS, and ES modules. No build step.
+
+`app/` is the whole application. It runs entirely in the browser: work in
+progress lives in the visitor's `localStorage`, and finished sheets and project
+files leave as downloads. **Nothing is written on the server**, and there is no
+endpoint that could write anything. That is what makes it safe to host without a
+login.
+
+Note this differs from the local development setup in the course folder, where
+`edit_server.py` accepts `POST /__save` and writes sheets straight to disk. That
+server is not deployed here, and must not be — reachable without a login it
+would be a write primitive for anyone who could load the page.
+
+## How it is served
+
+`workshop/src/routes/projects/exercise-builder/app/[...file]/+server.ts` reads
+this directory at request time. Consequences worth knowing:
+
+- **Updating the builder does not need a site rebuild.** Sync new files here and
+  a reload picks them up (responses are `no-cache` with an mtime-based ETag).
+  A rebuild is only needed when the landing page or the route itself changes.
+- Only recognised asset extensions are reachable, and paths are contained to
+  this directory — so nothing outside `app/` can be requested.
+- `.js` is served as `text/javascript`, without which browsers refuse ES
+  modules outright.
+- The route injects `<base href="/projects/exercise-builder/app/">` into the
+  served HTML, so the builder's relative URLs resolve whether or not the visitor
+  typed a trailing slash.
+
+Override the location with `EXERCISE_BUILDER_ROOT` in the workshop `.env` if it
+ever moves.
+
+## Updating
+
+Edit the files here. There is no build step and no sync — a reload picks the
+change up, for the reasons above.
+
+## Source of truth
+
+**This directory.** The builder was developed in the BI122G course folder on
+the workstation (`/Muspelheim/Employment/HiS/Teaching/BI122G`) and pushed here
+by `deploy_builder.sh`. That arrangement has ended and this installation is now
+canonical: **do not run `deploy_builder.sh` again**, as it would overwrite this
+directory with the older course-folder copy.
+
+The course folder still holds `test.mjs`, a Node test harness that was never
+deployed here — it needs `linkedom` and is no use in a browser.