[codex] add more live examples

[xinhen2117]

What changed

• Added Ethene (C2H4) and Acetylene (C2H2) to the sample molecule list in the main UI.
• Implemented builders for both molecules in the molecule editor so the examples render with the right atom counts and bond types.

Why this changed

Issue #8 asks for more live examples in the visualizer. These two common unsaturated molecules expand the demo set without changing the app flow.

Validation

• Ran npm run build in molecule-visualiser successfully.
• Build completed with pre-existing lint warnings only.