Fix: Python single-line comments incorrectly highlighting subsequent lines

[hygjob]

Bug Description

When rendering fenced Python code blocks, single-line comments starting with # would incorrectly cause the next line to be rendered with comment styling (dim fallback) instead of proper syntax highlighting.

Root Cause

syntect's HighlightLines::highlight_line() requires a trailing newline (\n) to properly terminate line-based scopes such as Python single-line comments. Without it, the comment scope leaks into the next line's highlighting context.

Changes

• src/renderer.rs: Append a trailing newline to each line before calling highlight_line(), then trim it from the resulting spans. Added a regression test (python_comment_does_not_affect_next_lines) to verify the fix.
• src/app.rs: Applied a minor clippy suggestion (or_else → or for Option references).

Verification

cargo test python_comment_does_not_affect_next_lines
New test passes, and existing tests continue to pass.