fix(web): keep the About mureo tab pinned as the last dashboard nav item

[hyoshi]

Summary

Follow-up to #229: keep the "About mureo" tab pinned as the last dashboard nav item, even when web-extension plugins (e.g. the agency extension) register their own tabs.

Previously the static nav order was setup/demo/byod/about/danger and extension tabs were appended at the end of the nav list, so any installed plugin tab landed below About.

Changes

• mureo/_data/web/app.html: static nav order is now setup / demo / byod / danger / about.
• mureo/_data/web/extensions.js: _renderNavItem inserts extension tabs before the About item (nav.insertBefore(li, aboutItem) via closest("li") on the [data-dashboard-nav="about"] anchor), falling back to the previous appendChild when the About item is absent. Extension tabs therefore slot between Danger Zone and About, however many plugins register one.
• tests/test_web_about.py: two new static pins — the last data-dashboard-nav key in app.html is about, and extensions.js carries the about-anchored insertBefore wiring.

No nav-order-dependent code exists elsewhere (verified: no :last-child/nextSibling/index-based selectors target the dashboard nav; BUILTIN_TABS and DEFAULT_NAV are position-independent). DOM order now matches visual order, so keyboard Tab reaches About last as well.

Test plan

• tests/test_web_about.py — 17 pass (incl. the two new order pins, TDD red→green)
• 216 pass across test_web_about / test_web_handlers / test_web_static_serving / test_web_no_framework; ruff + black clean
• CI green on the PR