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.

README.md

@@ -86,6 +86,11 @@ teachers, rooms, cancellations dropped and substitutions marked. Each heading is
 indexed as its own lesson, so a search answers "my own note, Deutsch,
 18.09.2026" rather than "Friday".
 
+Writing is **formatted, not Markdown**: headings, bold, lists, tick boxes,
+quotes, links and tables come from a toolbar, and `MD` shows the Markdown
+underneath when you want it. The file on disk stays Markdown either way — that
+is what the index reads and what outlives the app.
+
 It saves as you type, keeps a local copy of every keystroke for when the signal
 goes, and refuses a save that would overwrite a version it never saw. On a phone
 it adds to the home screen and opens standalone.