Write notes as formatted text, store them as Markdown

The editor was a textarea holding raw Markdown, which is the wrong thing
to hand someone taking notes during a lesson: nobody types `##` and `**`
while a teacher is talking. It now shows the note formatted and puts a
toolbar above it — headings, bold, lists, tick boxes, quotes, links,
tables — while the file on disk stays exactly what it was, because that
is what the indexer reads and what outlives this app.

`markdown.js` is the whole translation: `markdownToHtml` on the way in,
`markdownFromDom` on the way out. The property that matters is that the
round trip settles — one pass may tidy a note, a second must change
nothing — because these notes are the only record of what was said in
the room and there is nothing to restore a lossy save from. `editor.js`
checks exactly that before opening a note formatted, and a note it
cannot hold unchanged opens in the Markdown view and says so instead of
being quietly reduced.

No editor library: the content security policy allows no outside script
and the app has no bundler, so this is `contenteditable` and
`execCommand` with a tolerant serializer behind it — an element it does
not model keeps its words and loses its tag. Pasted HTML is converted to
Markdown before it reaches the document, which is the one place where
sanitising and formatting are the same operation.

Tested against `test/mini-dom.ts`, sixty lines of read-only DOM, rather
than a headless browser or a DOM dependency; the toolbar itself was
driven by hand in Firefox. WebKit has still never run it.

docs/DEPLOY-NOTES.md

@@ -12,7 +12,7 @@ should not make twice**, because changing it later means moving files by hand.
 | | |
 |---|---|
 | **Your own lesson notes** | A directory of Markdown files the server reads, indexes and searches beside Schulcloud and WebUntis. Three tools: `list_notes`, `get_note`, `add_note`. |
-| **The app at `/app`** | A login, a day-at-a-time notes editor, and a settings page that replaces the Schulcloud token. Only served when `WEB_PASSWORD` is set. |
+| **The app at `/app`** | A login, a day-at-a-time notes editor with a formatting toolbar, and a settings page that replaces the Schulcloud token. Only served when `WEB_PASSWORD` is set. |
 | **The WebUntis class register** | `untis_lesson_topics` now takes a subject as well as a period id, and `UNTIS_HISTORY_DAYS` of "what was actually taught" goes into the search index. |
 
 Nothing here changes Schulcloud or WebUntis: both stay read-only. The notes
@@ -241,6 +241,8 @@ write them.
 | `Der Server nimmt keine Änderungen an` | `NOTES_READONLY` is on | Remove it and recreate the container |
 | Saves refused as a conflict, repeatedly | The note is being changed elsewhere — a sync tool, another device | Choose a version in the banner; if a sync tool keeps rewriting the file, it is fighting the app |
 | `EACCES` for `/data/notes` in the logs | A bind-mounted directory the container's user cannot write | `sudo chown -R 1000:1000 /home/pi/Notizen` (match the image's user), then recreate |
+| The toolbar is there, the text stays plain | The browser blocked `editor.js` or `markdown.js` | Check the console; both must be served from `/app/`, and `app.js` must load as `type="module"` |
+| A note opens in the Markdown view by itself, with a hint | It holds formatting the formatted view cannot keep unchanged | Nothing is wrong and nothing was lost; edit it there, or simplify the note |
 | Notes exist but `search` cannot find them | Only a full crawl reads them | `schulcloud refresh --force` |
 | `search` finds a day note but names no subject | The lesson headings were rewritten past recognition | Keep `## 1. Deutsch …`; the leading number and the subject are what the index reads |
 | `untis_lesson_topics` with a subject finds nothing | The subject code differs from what you typed | Check it against `untis_timetable`; the register uses the school's own codes |