Fix broken "Share on LinkedIn" and "Copy Link" buttons on blog posts and blog list

[h30s]

Problem

When reading blog posts or browsing the blog list, the social share panel has buttons for X/Twitter, Facebook, LinkedIn, and a "Copy Link" button. However, the LinkedIn and Copy Link buttons do not work:

1. Share on LinkedIn: The button is hardcoded with href="#linkedinshare" instead of a LinkedIn sharing URL endpoint. There is no JavaScript handler for this ID, so clicking it just appends #linkedinshare to the browser URL hash without opening a LinkedIn share dialog.
2. Copy Link: The button has class="link link_yank" and links to the page URL, but there is no click handler attached to copy the URL to the clipboard. Clicking it just re-navigates/reloads the page instead of copying the link.

User Impact

• Readers cannot share OSL blog posts directly to LinkedIn.
• Users who click "Copy Link" expecting to copy the URL to their clipboard to share with others get an unnecessary page jump/reload, and nothing is copied.

Expected Behavior

• Clicking the LinkedIn share button should open the LinkedIn sharing dialog (e.g. https://www.linkedin.com/sharing/share-offsite/?url=<canonical_url>) in a new tab.
• Clicking the "Copy Link" button should copy the post's URL to the clipboard and provide visual feedback (e.g., changing the tooltip/text to "Copied!").

Relevant Code References

• theme/blog-post.html (lines 61–66):

  <a href="#linkedinshare" id="linkedinshare" class="linkedin" title="Share on LinkedIn" rel="nofollow">
    <svg class="icon" aria-hidden="true"><use xlink:href="#linkedin"/></svg>
  </a>
  <a href="{{ page.canonical_url or url }}" title="Copy link" class="link link_yank">
    <svg class="icon" aria-hidden="true"><use xlink:href="#copy"/></svg>
  </a>

• theme/blog-list.html (lines 66–71):

  <a href="#linkedinshare" class="linkedin" title="Share on LinkedIn" rel="nofollow">
    <svg class="icon"><use xlink:href="#linkedin"/></svg>
  </a>
  <a href="{{ url }}" title="Copy Link" class="link link_yank">
    <svg class="icon"><use xlink:href="#copy"/></svg>
  </a>