fix(notes): highlight markdown markup in the editor as documented

[sorenwacker]

The detail panel guide promised Markdown syntax highlighting in Edit and Split mode, but the editor registered no highlight style, so every character rendered plain. In the proportional UI font the asterisk glyph is lighter and smaller than the letters, so **Goals** looked like four oddly styled stars around a word instead of markup.

The editor now applies a highlight style with fixed class names (utils/markdownHighlight.js): markers (**, *, #, -, >, backticks, link brackets, table pipes) are dimmed to the tertiary text colour and never bold; bold text is bold, italic italic, headings bold, code monospace, links and URLs in the accent colour. Colours come from the theme variables, so light and dark themes both work.

@codemirror/language and @lezer/highlight were already installed transitively and are now declared as direct dependencies, bounded below the next major. The lockfile gained only those two entries.

Docs: the guide now states what is highlighted, and the claim that code blocks get language-specific colouring in Preview is removed because nothing implements it.

Tests: notesEditorHighlighting.test.js mounts the editor and checks the marker and content classes, plus the dependency declarations. All three failed before the change. Full suite, eslint, prettier and vue-tsc pass.