Feed links are followed only when they are web addresses (#326)

An item's link and an enclosure's URL are the publisher's, stored as
written, and the page put them in href and window.open through esc(),
which escapes characters, not schemes: a javascript: or data: link
survived as Open the original, View in a new tab (for a file not on
disk), the Directory's item links and the v key. Show notes already
lose those schemes to ammonia; these four sinks were the exception.

webUrl() passes http(s) only, and each sink goes through it. A link
that fails it shows no link, as an item with none does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/items.ts

@@ -415,7 +415,7 @@ function detailHtml(e: Entry){
         aria-label="Mark ${e.read?'unread':'read'}" aria-pressed="${!!e.read}">${e.read?ICON.check:ICON.unread}</button>
       <button class="btn ico" data-a="flag" title="${e.flagged?'Pinned: never deleted. Unpin':'Pin, so it is never deleted'}"
         aria-label="${e.flagged?'Unpin':'Pin'}" aria-pressed="${!!e.flagged}">${e.flagged?ICON.pinOn:ICON.pin}</button>
-      ${e.link?`<a class="btn ico" href="${esc(e.link)}" target="_blank" rel="noopener noreferrer"
+      ${webUrl(e.link)?`<a class="btn ico" href="${esc(webUrl(e.link))}" target="_blank" rel="noopener noreferrer"
         title="Open the original" aria-label="Open the original">${ICON.open}</a>
       <button class="btn ico" data-a="share" title="Share" aria-label="Share">${ICON.share}</button>`:''}</span>
     </div>
@@ -476,7 +476,7 @@ function encBox(x: Enclosure){
   // Nothing on disk. For an image or a PDF you usually just want to look at it, so link
   // straight to the publisher's copy in a new tab -- no download, and nothing proxied
   // through here, which would make ipx a fetch-anything relay.
-  const viewable = !isPlayable(x) && x.state !== 'pending' && x.state !== 'held';
+  const viewable = !!webUrl(x.url) && !isPlayable(x) && x.state !== 'pending' && x.state !== 'held';
   return `<div class="encbox">
     ${kindIcon(x)}
     <span class="meta" style="flex:1">${size}</span>