fix: remove duplicate scroll-top-btn on project page (issue #1878)

[ionfwsrijan]

Problem

project.html rendered two buttons with id="scroll-top-btn" — an inline ↑ button followed by the partials/scroll_top_btn.html include that defines the same ID. Duplicate IDs are invalid HTML, and document.getElementById("scroll-top-btn") in static/script.js resolves only to the first element, leaving the partial's button (and its expected behavior) inert.

Fix

• Removed the inline ↑ button from project.html and rely on the shared partials/scroll_top_btn.html include — the same pattern explore.html uses.
• Added tests/test_html_template_duplicate_ids.py — a pure-file CI sanity check that scans every template (including partials) and fails if any element ID is defined more than once, preventing duplicate-ID regressions.

Files changed

• src/templates/project.html — dropped the duplicate inline scroll button.
• tests/test_html_template_duplicate_ids.py — new duplicate-ID check across src/templates/**/*.html.

Testing

• tests/test_html_template_duplicate_ids.py — 1 passed (run with --noconftest since the app can't boot locally due to the pre-existing App fails to boot: NameError: name 're' is not defined in portfolio_analyzer.py #1810).
• Verified document.getElementById("scroll-top-btn") in static/script.js now targets the single button from the partial.

Closes #1878

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