Share a feed, an item or a file (#48)

A share button in the feed's header, beside an item's "Open the original", and on each file.
It uses the Web Share API where the browser has it, which is the share sheet on a phone, and
copies the link where it does not. A file is shared by the publisher's address, not /media/,
which only someone signed in here can open.

Paid feeds carry the subscriber's access in their address, Patreon's in a token, so sharing one
gives the subscription away. An address that looks like that asks first.

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

CHANGELOG.md

@@ -13,6 +13,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   list of its own; an item with one of those words or phrases in its title or text is hidden
   from you and is not downloaded on your account. Other people's lists never hide anything from
   you.
+- Share buttons for a feed, an item and a file. On a phone they open the system's share sheet;
+  elsewhere they copy the link. A link that looks like it carries your own access to a paid
+  feed asks before it goes anywhere.
 
 ### Fixed