fix(docs[spa]): Fix prompt copy buttons surviving SPA navigation

[tony]

Summary

• Fix prompt copy buttons breaking after SPA navigation
• Replace clone-based button creation with inline HTML template matching sphinx-copybutton's exact output
• Switch prompt-copy.js from direct event binding to document-level event delegation (capture phase)
• Match sphinx-copybutton's exact success feedback (green checkmark SVG swap + staggered timeouts)

Root causes fixed

1. No buttons after SPA nav from index.html: addCopyButtons() cloned an existing .copybtn as template — but index.html has no code blocks, so the template was always null when landing there first. Replaced with inline HTML template.

2. Prompt blocks skipped: addCopyButtons() only created buttons for div.highlight pre (code blocks), not div.admonition.prompt > p:last-child (prompts). Added prompt block handling.

3. Event listeners destroyed by DOM swap: prompt-copy.js used direct addEventListener on each button — destroyed when SPA navigation replaced .article-container. Switched to document-level event delegation in capture phase.

4. Wrong button detection selector: .closest("div.admonition.prompt .copybtn") assumed button was a descendant of the prompt div. sphinx-copybutton inserts buttons via insertAdjacentHTML('afterend') making them siblings. Fixed by checking where data-clipboard-target points instead.

5. Missing success feedback: prompt-copy.js only did tooltip+class, not the SVG icon swap to green checkmark. Now matches sphinx-copybutton's exact behavior.

Test plan

• Load index.html, SPA-navigate to Quickstart via sidebar — copy buttons appear on all code blocks and prompt blocks
• Click a prompt copy button with inline code — green checkmark appears, clipboard contains markdown with backticks
• SPA-navigate away and back — prompt copy buttons still work
• Direct-load a page with prompts (e.g. /recipes/) — copy buttons work on first load
• Code block copy buttons still work identically before and after SPA nav

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.10%. Comparing base (ee213d1) to head (d36cb1c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #5   +/-   ##
=======================================
  Coverage   71.10%   71.10%           
=======================================
  Files          20       20           
  Lines        1419     1419           
  Branches      186      186           
=======================================
  Hits         1009     1009           
  Misses        365      365           
  Partials       45       45           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.