Integrate ETable/maketables rendering into GenerateReport pipeline step

[drbenvincent]

Context

PR #773 introduced the maketables plugin with ETable support, providing polished HTML table rendering for experiment results. PR #749 introduced the GenerateReport pipeline step, which renders HTML reports with effect summaries, plots, and sensitivity check results.

Currently GenerateReport (in causalpy/steps/report.py) renders all tables via plain DataFrame.to_html(). It has no awareness of the maketables / ETable integration introduced in #773.

Problem

Users who run a full pipeline workflow get a report with unstyled, raw HTML tables, even though the same experiment object supports rich ETable rendering elsewhere (e.g., in notebooks). The report should take advantage of the nicer formatting when maketables is available.

Proposed change

Update GenerateReport to optionally use ETable rendering for the effect summary table (and potentially sensitivity check tables) when maketables is installed. Since maketables is an optional dependency, the integration should degrade gracefully to the current DataFrame.to_html() path when it is not available.

Acceptance criteria

• GenerateReport uses ETable-style rendering for the effect summary when maketables is available.
• Falls back to DataFrame.to_html() when maketables is not installed.
• Report HTML template updated to accommodate the richer table markup.
• Existing tests continue to pass; new test covers the ETable rendering path.
• Pre-commit passes.