Task 217: Side-view SVG E2E tests + hydration script dead code cleanup

Status: DONE

Description

Two focused improvements:

1. E2E tests for side-view SVG diagrams (2 tests): Cross-episode page has Saturn ring

crossing and Uranus approach geometry side-view diagrams. Verify SVG renders with

expected elements (circle, line, ellipse, path, description paragraph).

1. Dead code cleanup: Removed unused mainIdx variable assignment in the uPlot

hydration script (templates.ts inline DOMContentLoaded handler). The variable was

assigned but never referenced.

Verification

• 1776 TS unit tests: ALL PASS
• 14 new E2E tests (KaTeX + transcription + sideview): ALL PASS
• Total E2E: 114 (was 112)