Fix index.html clean-URL rewrite mangling alternate-format links on index pages

[cscheid]

Closes #14667.

Problem

quarto-nav.js rewrites every link on page load to clean up index.html URLs, using an unanchored regex (/\/index\.html/). Any href that merely contains /index.html as a prefix gets mangled: on a site's index page, the "Other Formats" link to index.html.md (the markdown twin produced when output-file: index.html is paired with a markdown format — e.g. by nbdev for llms.txt workflows) was rewritten to /.md, which 404s. Live example in the issue: the CommonMark link on https://fastcore.fast.ai/.

Fix

Anchor the regex to the end of the path, still allowing a query or fragment:

links[i].href = links[i].href.replace(/\/index\.html(?=[?#]|$)/, "/");

href ends with…	before	after
/index.html	/	/ (same)
/index.html#sec	/#sec	/#sec (same)
/index.html?a=1	/?a=1	/?a=1 (same)
/index.html.md	/.md ❌	unchanged ✅
/index.html/foo	//foo ❌	unchanged ✅

The same unanchored pattern existed in two more places, fixed identically:

• itemHasNavTarget in src/project/types/website/website-shared.ts (navbar/sidebar active-state matching)
• the preview reload-target normalization in src/webui/quarto-preview/src/server/navigation.ts (checked-in bundle rebuilt with npm run build; one-line diff)

Tests

New playwright test (website-index-html-links.spec.ts + site under tests/docs/playwright/website/issue-14667/) covering all five cases above; passes in chromium/firefox/webkit and verified to fail against the old regex. The test site uses hand-written links rather than the issue's output-file: index.html + commonmark config because that config cannot currently render — that's a separate pre-existing bug, filed as #14669.

Also verified manually: rebuilt the issue's repro with this branch, served it over http, and confirmed in a browser that the CommonMark link keeps index.html.md while clean-URL rewriting and navbar active-state marking still behave (existing blog-simple-blog and html-toc-js specs still pass).

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