Only apply markdown decorations when the note has markdown enabled

[gunjanjaswal]

Fixes #3379.

Markdown decorations (headings, bold, italic, list markers, and so on) were applied in the editor even when the note had Markdown disabled. Every other Markdown behavior in note-content-editor.tsx gates on the note's markdown system tag, but the decoration path in setDecorators did not, so a plain note with a line like # comment or *.conf still got heading/emphasis styling. This is the v2.26.0 regression reported in #3379.

Changes proposed

• markdown-decorations.ts: getMarkdownDecorations now takes a markdownEnabled flag (defaults to true) and returns [] when it is false.
• note-content-editor.tsx: pass note.systemTags.includes('markdown'). The existing clear() stays unconditional, so decorations are removed when Markdown is toggled off on an open note.
• markdown-decorations.test.ts: add a case asserting no decorations are returned when markdown is disabled.

Testing instructions

npx jest lib/markdown-decorations.test.ts

Or manually: open a note with Markdown disabled and a body containing # heading or **bold**, confirm the text stays plain, then toggle Markdown on and confirm the styling appears.