fix: highlight backtick spans that start at the beginning of a line

[dpecos]

Summary

• Fixes the highlightSpans regex in slideView.ts so that backtick spans at the start of a line (or the start of the HTML string) are matched and highlighted correctly
• Applies the same fix to the custom RegExp path
• Adds a new test file highlightSpans.test.ts with 6 cases covering the bug scenario and surrounding edge cases

Root cause

// before — requires a non-backtick character before the opening backtick
pattern = /([^`])`([^`]+?)`/g;

// after — also allows start-of-line as the "preceding context"
pattern = /(^|[^`])`([^`]+?)`/gm;

The original ([^\])` group consumed exactly one character, so a span whose opening backtick was the first character on a line (or the first character in the string) never matched.

Fix

Changed the leading group to (^|[^\])and added them(multiline) flag so^matches the start of every line, not just the start of the full string. The replacement callback already handlese` being an empty string correctly — it simply prepends nothing.

Closes #2