Card + article-page polish

* Placeholder: bold, slightly larger initial letter on the topic word; make
  health (teal) and environment (leaf green) clearly distinct and show more hue
  in the deepened word so they're easy to tell apart.
* Article page: the source name was chopped to its first word ("Read the full
  story at The") — use the full publisher name; open the source link in a new
  tab so upbeatbytes.com stays put.
* Use the new SVG back arrow on the account and admin top bars (matching the
  article page) instead of the old "←" glyph.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/share.py

@@ -55,8 +55,6 @@ def render_share_page(article: dict, base_url: str, summary: str | None = None)
         for t in raw_tags.split(",") if t
     )
     groupings = f'<div class="chips">{chips}</div>' if chips else ""
-
-    source_short = source.split(" ")[0] if source else "the source"
     if summary:
         summary_block = f'<p class="summary" id="summary">{escape(summary)}</p>'
         poll = ""
@@ -162,7 +160,7 @@ def render_share_page(article: dict, base_url: str, summary: str | None = None)
         <p class="why"><span class="lbl">Why it's here</span>{escape(why)}</p>
         {groupings}
         <div class="actions">
-          <a class="primary" href="{escape(src_url)}" rel="noopener" data-src-click>Read the full story at {escape(source_short)}</a>
+          <a class="primary" href="{escape(src_url)}" target="_blank" rel="noopener" data-src-click>Read the full story at {escape(source)}</a>
           <button class="secondary" type="button" data-share>Copy link</button>
           <a class="secondary" href="/">Explore Upbeat Bytes →</a>
         </div>