Fix empty accessibility element cards

[aguspe]

Summary

• Fix node parsing regexes to handle end-of-string (not just newlines) — the last element in each violation block had no trailing newline, causing empty cards
• Support "Fix all of the following:" in addition to "Fix any of the following:" format
• Fall back to grabbing the indented description line after HTML when no explicit "Fix" block exists
• Filter out empty nodes so blank cards never render

Test plan

• All 577 unit tests pass
• TypeScript typecheck passes
• Verify all 3 elements in color-contrast violation now show selector and description
• Verify heading-order violation elements display correctly

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