fix: recognise ??? notes separator with trailing whitespace

[dpecos]

Summary

• One-character regex fix in lexer.ts: adds [ \t]* before the end anchor of the NOTES_SEPARATOR pattern
• Adds 2 lexer tests (trailing spaces, trailing tab)
• Adds 2 parser-level tests confirming notes are correctly split from content in both cases

Root cause

// before
NOTES_SEPARATOR: /(?:^|\n)(\?{3})(?:\n|$)/,

// after
NOTES_SEPARATOR: /(?:^|\n)(\?{3})[ \t]*(?:\n|$)/,

Any trailing space or tab on the ??? line prevented the regex from matching, so the separator was not recognised and all notes were rendered as regular slide content.

Closes #1