Release 0.5.3: OPML orphan scan, feed error UI, small UI fixes

- Stop scanning an OPML/Patreon feed's derived rows once nobody subscribes
  to it; retire them (drop or orphan) the way sync_group already does when
  the list itself drops one. This is what let 922 defunct davewiner feeds
  keep scanning hourly after the OPML left config.
- Repair feed XML with a bare `&`, and give a plain reason (moved web page
  with its new address when linked, or nothing yet for an empty body)
  instead of a raw parser error.
- Show a failing feed's plain-English reason and next step (Unsubscribe /
  Use the new address) in the sidebar and on its own page, once it has
  been down a day.
- Fix four small UI bugs: show-note links open in a new tab, video files
  play as video, an opened item no longer disappears from the Unread tab,
  and Subscribe/Unsubscribe get their own icons.
- Fix Settings disappearing for non-admin accounts: it was hiding the
  whole modal instead of just the admin-only parts (Users, the editable
  schedule/quota, Save), which are the only parts the server actually
  refuses them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DmQfE1eFPApnXWyPHBWqUA

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ipx"
-version = "0.5.2"
+version = "0.5.3"
 edition = "2024"
 
 [dependencies]