fix(editor): remove underline beneath markdown h1/h2 headings

[AmethystLiang]

Summary

• H2 headings in markdown preview and rich editor rendered with a thin border-bottom rule beneath them
• Exported H1 and H2 headings had the same underline treatment via hardcoded styles in export-css.ts
• Removes border-bottom and padding-bottom from all three locations so headings sit flush with following content

Test plan

• Open a markdown note with H1 and H2 headings — confirm no underline appears in preview
• Open a note in rich editor mode — confirm H2 headings have no bottom border
• Export a note to HTML/PDF — confirm exported H1/H2 headings have no underline

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